diff --git a/packages/grpc-js/src/transport.ts b/packages/grpc-js/src/transport.ts
index ba0675d08..3767b4cf0 100644
--- a/packages/grpc-js/src/transport.ts
+++ b/packages/grpc-js/src/transport.ts
@@ -719,14 +719,33 @@ export class Http2SubchannelConnector implements SubchannelConnector {
         settings: {
           initialWindowSize:
             options['grpc-node.flow_control_window'] ??
-            http2.getDefaultSettings().initialWindowSize,
+            http2.getDefaultSettings().initialWindowSize ?? 65535,
         }
       });
+
+      // Prepare window size configuration for remoteSettings handler
+      const defaultWin = http2.getDefaultSettings().initialWindowSize ?? 65535; // 65 535 B
+      const connWin = options[
+        'grpc-node.flow_control_window'
+      ] as number | undefined;
+
       this.session = session;
       let errorMessage = 'Failed to connect';
       let reportedError = false;
       session.unref();
       session.once('remoteSettings', () => {
+        // Send WINDOW_UPDATE now to avoid 65 KB start-window stall.
+        if (connWin && connWin > defaultWin) {
+          try {
+            // Node ≥ 14.18
+            (session as any).setLocalWindowSize(connWin);
+          } catch {
+            // Older Node: bump by the delta
+            const delta = connWin - (session.state.localWindowSize ?? defaultWin);
+            if (delta > 0) (session as any).incrementWindowSize(delta);
+          }
+        }
+        
         session.removeAllListeners();
         secureConnectResult.socket.removeListener('close', closeHandler);
         secureConnectResult.socket.removeListener('error', errorHandler);