diff --git a/packages/express/src/snapshot/zod/v1/dtos/mappers.ts b/packages/express/src/snapshot/zod/v1/dtos/mappers.ts
index 60e55e7..05cfcdb 100644
--- a/packages/express/src/snapshot/zod/v1/dtos/mappers.ts
+++ b/packages/express/src/snapshot/zod/v1/dtos/mappers.ts
@@ -42,10 +42,12 @@ export function mapFromAllMapsDto(dto: dtos.AllMapsDto): types.AllMaps {
 export function mapFromComplexKeyMapADto(
   dto: dtos.ComplexKeyMapADto,
 ): types.ComplexKeyMapA {
-  return Object.keys(dto).reduce((acc, key) => {
+  const result: types.ComplexKeyMapA = {};
+  for (const key of Object.keys(dto)) {
     const value = dto[key];
-    return value === undefined ? acc : { ...acc, [key]: value };
-  }, {} as types.ComplexKeyMapA);
+    if (value !== undefined) result[key] = value;
+  }
+  return result;
 }
 
 export function mapFromCreateWidgetBodyDto(
@@ -159,13 +161,15 @@ export function mapFromMixedMapDDto(dto: dtos.MixedMapDDto): types.MixedMapD {
 }
 
 export function mapFromPureMapADto(dto: dtos.PureMapADto): types.PureMapA {
-  return Object.keys(dto).reduce((acc, key) => {
+  const result: types.PureMapA = {};
+  for (const key of Object.keys(dto)) {
     const value =
       typeof dto[key] === 'undefined'
         ? dto[key]
         : mapFromExampleMapValueDto(dto[key] as dtos.ExampleMapValueDto);
-    return value === undefined ? acc : { ...acc, [key]: value };
-  }, {} as types.PureMapA);
+    if (value !== undefined) result[key] = value;
+  }
+  return result;
 }
 
 export function mapFromPureMapBDto(dto: dtos.PureMapBDto): types.PureMapB {
@@ -176,13 +180,15 @@ export function mapFromPureMapBDto(dto: dtos.PureMapBDto): types.PureMapB {
     objBuzz: mapFromExampleMapValueDto(objBuzz),
   });
 
-  return Object.keys(__rest__).reduce((acc, key) => {
+  const result: types.PureMapB = __defined__;
+  for (const key of Object.keys(__rest__)) {
     const value =
       typeof dto[key] === 'undefined'
         ? dto[key]
         : mapFromExampleMapValueDto(dto[key] as dtos.ExampleMapValueDto);
-    return value === undefined ? acc : { ...acc, [key]: value };
-  }, __defined__ as types.PureMapB);
+    if (value !== undefined) result[key] = value;
+  }
+  return result;
 }
 
 export function mapFromPureMapCDto(dto: dtos.PureMapCDto): types.PureMapC {
@@ -193,13 +199,15 @@ export function mapFromPureMapCDto(dto: dtos.PureMapCDto): types.PureMapC {
     objBuzz: mapFromExampleMapValueDto(objBuzz),
   });
 
-  return Object.keys(__rest__).reduce((acc, key) => {
+  const result: types.PureMapC = __defined__;
+  for (const key of Object.keys(__rest__)) {
     const value =
       typeof dto[key] === 'undefined'
         ? dto[key]
         : mapFromExampleMapValueDto(dto[key] as dtos.ExampleMapValueDto);
-    return value === undefined ? acc : { ...acc, [key]: value };
-  }, __defined__ as types.PureMapC);
+    if (value !== undefined) result[key] = value;
+  }
+  return result;
 }
 
 export function mapFromPureMapDDto(dto: dtos.PureMapDDto): types.PureMapD {
@@ -228,10 +236,12 @@ export function mapToAllMapsDto(obj: types.AllMaps): dtos.AllMapsDto {
 export function mapToComplexKeyMapADto(
   obj: types.ComplexKeyMapA,
 ): dtos.ComplexKeyMapADto {
-  return Object.keys(obj).reduce((acc, key) => {
+  const result: dtos.ComplexKeyMapADto = {};
+  for (const key of Object.keys(obj)) {
     const value = obj[key];
-    return value === undefined ? acc : { ...acc, [key]: value };
-  }, {} as dtos.ComplexKeyMapADto);
+    if (value !== undefined) result[key] = value;
+  }
+  return result;
 }
 
 export function mapToExampleMapValueDto(
@@ -346,13 +356,15 @@ export function mapToMixedMapDDto(obj: types.MixedMapD): dtos.MixedMapDDto {
 }
 
 export function mapToPureMapADto(obj: types.PureMapA): dtos.PureMapADto {
-  return Object.keys(obj).reduce((acc, key) => {
+  const result: dtos.PureMapADto = {};
+  for (const key of Object.keys(obj)) {
     const value =
       typeof obj[key] === 'undefined'
         ? obj[key]
         : mapToExampleMapValueDto(obj[key] as types.ExampleMapValue);
-    return value === undefined ? acc : { ...acc, [key]: value };
-  }, {} as dtos.PureMapADto);
+    if (value !== undefined) result[key] = value;
+  }
+  return result;
 }
 
 export function mapToPureMapBDto(obj: types.PureMapB): dtos.PureMapBDto {
@@ -363,13 +375,15 @@ export function mapToPureMapBDto(obj: types.PureMapB): dtos.PureMapBDto {
     'obj-buzz': mapToExampleMapValueDto(objBuzz),
   });
 
-  return Object.keys(__rest__).reduce((acc, key) => {
+  const result: dtos.PureMapBDto = __defined__;
+  for (const key of Object.keys(__rest__)) {
     const value =
       typeof obj[key] === 'undefined'
         ? obj[key]
         : mapToExampleMapValueDto(obj[key] as types.ExampleMapValue);
-    return value === undefined ? acc : { ...acc, [key]: value };
-  }, __defined__ as dtos.PureMapBDto);
+    if (value !== undefined) result[key] = value;
+  }
+  return result;
 }
 
 export function mapToPureMapCDto(obj: types.PureMapC): dtos.PureMapCDto {
@@ -380,13 +394,15 @@ export function mapToPureMapCDto(obj: types.PureMapC): dtos.PureMapCDto {
     'obj-buzz': mapToExampleMapValueDto(objBuzz),
   });
 
-  return Object.keys(__rest__).reduce((acc, key) => {
+  const result: dtos.PureMapCDto = __defined__;
+  for (const key of Object.keys(__rest__)) {
     const value =
       typeof obj[key] === 'undefined'
         ? obj[key]
         : mapToExampleMapValueDto(obj[key] as types.ExampleMapValue);
-    return value === undefined ? acc : { ...acc, [key]: value };
-  }, __defined__ as dtos.PureMapCDto);
+    if (value !== undefined) result[key] = value;
+  }
+  return result;
 }
 
 export function mapToPureMapDDto(obj: types.PureMapD): dtos.PureMapDDto {
@@ -419,13 +435,15 @@ export function mapToWidgetDataDto(obj: types.WidgetData): dtos.WidgetDataDto {
     buzz: mapToGizmoMapValueDto(buzz),
   });
 
-  return Object.keys(__rest__).reduce((acc, key) => {
+  const result: dtos.WidgetDataDto = __defined__;
+  for (const key of Object.keys(__rest__)) {
     const value =
       typeof obj[key] === 'undefined'
         ? obj[key]
         : mapToGizmoMapValueDto(obj[key] as types.GizmoMapValue);
-    return value === undefined ? acc : { ...acc, [key]: value };
-  }, __defined__ as dtos.WidgetDataDto);
+    if (value !== undefined) result[key] = value;
+  }
+  return result;
 }
 
 export function mapToWidgetFooDto(obj: types.WidgetFoo): dtos.WidgetFooDto {
diff --git a/packages/http-client/src/snapshot/zod/v1/dtos/mappers.ts b/packages/http-client/src/snapshot/zod/v1/dtos/mappers.ts
index 842446c..e4561da 100644
--- a/packages/http-client/src/snapshot/zod/v1/dtos/mappers.ts
+++ b/packages/http-client/src/snapshot/zod/v1/dtos/mappers.ts
@@ -42,10 +42,12 @@ export function mapFromAllMapsDto(dto: dtos.AllMapsDto): types.AllMaps {
 export function mapFromComplexKeyMapADto(
   dto: dtos.ComplexKeyMapADto,
 ): types.ComplexKeyMapA {
-  return Object.keys(dto).reduce((acc, key) => {
+  const result: types.ComplexKeyMapA = {};
+  for (const key of Object.keys(dto)) {
     const value = dto[key];
-    return value === undefined ? acc : { ...acc, [key]: value };
-  }, {} as types.ComplexKeyMapA);
+    if (value !== undefined) result[key] = value;
+  }
+  return result;
 }
 
 export function mapFromExampleMapValueDto(
@@ -177,13 +179,15 @@ export function mapFromMixedMapDDto(dto: dtos.MixedMapDDto): types.MixedMapD {
 }
 
 export function mapFromPureMapADto(dto: dtos.PureMapADto): types.PureMapA {
-  return Object.keys(dto).reduce((acc, key) => {
+  const result: types.PureMapA = {};
+  for (const key of Object.keys(dto)) {
     const value =
       typeof dto[key] === 'undefined'
         ? dto[key]
         : mapFromExampleMapValueDto(dto[key] as dtos.ExampleMapValueDto);
-    return value === undefined ? acc : { ...acc, [key]: value };
-  }, {} as types.PureMapA);
+    if (value !== undefined) result[key] = value;
+  }
+  return result;
 }
 
 export function mapFromPureMapBDto(dto: dtos.PureMapBDto): types.PureMapB {
@@ -194,13 +198,15 @@ export function mapFromPureMapBDto(dto: dtos.PureMapBDto): types.PureMapB {
     objBuzz: mapFromExampleMapValueDto(objBuzz),
   });
 
-  return Object.keys(__rest__).reduce((acc, key) => {
+  const result: types.PureMapB = __defined__;
+  for (const key of Object.keys(__rest__)) {
     const value =
       typeof dto[key] === 'undefined'
         ? dto[key]
         : mapFromExampleMapValueDto(dto[key] as dtos.ExampleMapValueDto);
-    return value === undefined ? acc : { ...acc, [key]: value };
-  }, __defined__ as types.PureMapB);
+    if (value !== undefined) result[key] = value;
+  }
+  return result;
 }
 
 export function mapFromPureMapCDto(dto: dtos.PureMapCDto): types.PureMapC {
@@ -211,13 +217,15 @@ export function mapFromPureMapCDto(dto: dtos.PureMapCDto): types.PureMapC {
     objBuzz: mapFromExampleMapValueDto(objBuzz),
   });
 
-  return Object.keys(__rest__).reduce((acc, key) => {
+  const result: types.PureMapC = __defined__;
+  for (const key of Object.keys(__rest__)) {
     const value =
       typeof dto[key] === 'undefined'
         ? dto[key]
         : mapFromExampleMapValueDto(dto[key] as dtos.ExampleMapValueDto);
-    return value === undefined ? acc : { ...acc, [key]: value };
-  }, __defined__ as types.PureMapC);
+    if (value !== undefined) result[key] = value;
+  }
+  return result;
 }
 
 export function mapFromPureMapDDto(dto: dtos.PureMapDDto): types.PureMapD {
@@ -253,13 +261,15 @@ export function mapFromWidgetDataDto(
     buzz: mapFromGizmoMapValueDto(buzz),
   });
 
-  return Object.keys(__rest__).reduce((acc, key) => {
+  const result: types.WidgetData = __defined__;
+  for (const key of Object.keys(__rest__)) {
     const value =
       typeof dto[key] === 'undefined'
         ? dto[key]
         : mapFromGizmoMapValueDto(dto[key] as dtos.GizmoMapValueDto);
-    return value === undefined ? acc : { ...acc, [key]: value };
-  }, __defined__ as types.WidgetData);
+    if (value !== undefined) result[key] = value;
+  }
+  return result;
 }
 
 export function mapFromWidgetFooDto(dto: dtos.WidgetFooDto): types.WidgetFoo {
@@ -286,10 +296,12 @@ export function mapToAllMapsDto(obj: types.AllMaps): dtos.AllMapsDto {
 export function mapToComplexKeyMapADto(
   obj: types.ComplexKeyMapA,
 ): dtos.ComplexKeyMapADto {
-  return Object.keys(obj).reduce((acc, key) => {
+  const result: dtos.ComplexKeyMapADto = {};
+  for (const key of Object.keys(obj)) {
     const value = obj[key];
-    return value === undefined ? acc : { ...acc, [key]: value };
-  }, {} as dtos.ComplexKeyMapADto);
+    if (value !== undefined) result[key] = value;
+  }
+  return result;
 }
 
 export function mapToCreateWidgetBodyDto(
@@ -386,13 +398,15 @@ export function mapToMixedMapDDto(obj: types.MixedMapD): dtos.MixedMapDDto {
 }
 
 export function mapToPureMapADto(obj: types.PureMapA): dtos.PureMapADto {
-  return Object.keys(obj).reduce((acc, key) => {
+  const result: dtos.PureMapADto = {};
+  for (const key of Object.keys(obj)) {
     const value =
       typeof obj[key] === 'undefined'
         ? obj[key]
         : mapToExampleMapValueDto(obj[key] as types.ExampleMapValue);
-    return value === undefined ? acc : { ...acc, [key]: value };
-  }, {} as dtos.PureMapADto);
+    if (value !== undefined) result[key] = value;
+  }
+  return result;
 }
 
 export function mapToPureMapBDto(obj: types.PureMapB): dtos.PureMapBDto {
@@ -403,13 +417,15 @@ export function mapToPureMapBDto(obj: types.PureMapB): dtos.PureMapBDto {
     'obj-buzz': mapToExampleMapValueDto(objBuzz),
   });
 
-  return Object.keys(__rest__).reduce((acc, key) => {
+  const result: dtos.PureMapBDto = __defined__;
+  for (const key of Object.keys(__rest__)) {
     const value =
       typeof obj[key] === 'undefined'
         ? obj[key]
         : mapToExampleMapValueDto(obj[key] as types.ExampleMapValue);
-    return value === undefined ? acc : { ...acc, [key]: value };
-  }, __defined__ as dtos.PureMapBDto);
+    if (value !== undefined) result[key] = value;
+  }
+  return result;
 }
 
 export function mapToPureMapCDto(obj: types.PureMapC): dtos.PureMapCDto {
@@ -420,13 +436,15 @@ export function mapToPureMapCDto(obj: types.PureMapC): dtos.PureMapCDto {
     'obj-buzz': mapToExampleMapValueDto(objBuzz),
   });
 
-  return Object.keys(__rest__).reduce((acc, key) => {
+  const result: dtos.PureMapCDto = __defined__;
+  for (const key of Object.keys(__rest__)) {
     const value =
       typeof obj[key] === 'undefined'
         ? obj[key]
         : mapToExampleMapValueDto(obj[key] as types.ExampleMapValue);
-    return value === undefined ? acc : { ...acc, [key]: value };
-  }, __defined__ as dtos.PureMapCDto);
+    if (value !== undefined) result[key] = value;
+  }
+  return result;
 }
 
 export function mapToPureMapDDto(obj: types.PureMapD): dtos.PureMapDDto {
diff --git a/packages/typescript-dtos/src/mapper-factory.ts b/packages/typescript-dtos/src/mapper-factory.ts
index bdfdd99..70157eb 100644
--- a/packages/typescript-dtos/src/mapper-factory.ts
+++ b/packages/typescript-dtos/src/mapper-factory.ts
@@ -243,6 +243,7 @@ export class ExpressMapperFactory extends BaseFactory {
         mode,
       ).input;
 
+      // TODO: remove O(n²) call to reduce
       yield ``;
       yield `return Object.keys(__rest__).reduce((acc, key) => {`;
       yield `const value = ${this.builder.buildValue(mapProperties.value.value, mode, `${paramName}[key]`, asType)};`;
@@ -350,10 +351,12 @@ export class ExpressMapperFactory extends BaseFactory {
       const initialObj = destructure ? '__defined__' : '{}';
 
       yield ``;
-      yield `return Object.keys(${sourceName}).reduce((acc, key) => {`;
-      yield `const value = ${this.builder.buildValue(mapProperties.value.value, mode, `${paramName}[key]`, asType)};`;
-      yield `return value === undefined ? acc : { ...acc, [key]: value };`;
-      yield `}, ${initialObj} as ${this.buildTypeNames(type.name.value, mode).output});`;
+      yield `const result: ${this.buildTypeNames(type.name.value, mode).output} = ${initialObj};`;
+      yield `for (const key of Object.keys(${sourceName})) {`;
+      yield `  const value = ${this.builder.buildValue(mapProperties.value.value, mode, `${paramName}[key]`, asType)};`;
+      yield `  if (value !== undefined) result[key] = value;`;
+      yield `}`;
+      yield `return result;`;
     }
   }
 
@@ -716,6 +719,7 @@ export class ExpressMapperFactory extends BaseFactory {
     this._needsCompact = true;
   }
   private *buildCompact(): Iterable<string> {
+    // TODO: remove O(n²) call to reduce
     if (this._needsCompact) {
       yield `function compact<T extends object>(obj: T): T {
         return Object.keys(obj).reduce((acc, key) => {