Parachain bootstrap waits up to 67s for relay finalization

[BigTava]

Problem

From 30 cold-start runs against Paseo, Asset Hub Paseo, smoldot 3.1.0 via polkadot-api/smoldot, browser, fully cold (no IndexedDB):

Phase	Median	p90	Max
Warp sync (GRANDPA fragment verification)	2 s	2.6 s	2.9 s
Wait for first relay-chain finalization	17 s	51 s	67 s
Parachain bootstrap (runtime + Aura proofs)	2.4 s	4.8 s	13 s
Asset Hub header (compare bootstrap height and relay's current finalized parahead)	2.1 s	4.3 s	10 s

Proposal

At the consumer side (e.g. parachain.rs:1129) accept Notification::BestBlockChanged / Block { is_new_best } in addition to Finalized. Trades strict-finality on the first useful block for lower latency; reconciliation when a real finalized notification later arrives.

Related

• Reduce browser boot time: ~7-8s warm restart to <2s for dotNS resolution #3153 — umbrella for browser cold-start (warm-restart variant)
• fix: parachain warm-restart hang #3210 — initial finalized block (warm-start fix; cold path has the same shape)
• perf(sync-service): skip parachain runtime download on warm start #3214 — skip parachain runtime download (warm)
• perf(sync-service): parallelize Aura call proofs during parachain bootstrap #3219 / perf(sync-service): overlap WASM compilation with Aura proof downloads #3220 / refactor(sync-service): convert start_parachain from blocking linear function to reactive state machine #3222 — parachain bootstrap perf, downstream of this wait

[BigTava]

Re-ran same dotli resolution test with #3236.

Phase	Median	p90	Max
Warp sync (GRANDPA fragment verification)	2.4 s	2.5 s	3.2 s
Wait for first relay-chain finalization	0.2 s	0.2 s	0.8 s
Parachain bootstrap (runtime + Aura proofs)	1.5 s	2.4 s	2.8 s
Asset Hub header (compare bootstrap height and relay's current finalized parahead)	0.6 s	1.5 s	2.4 s

[skunert]

Closing this in favor of #3237, which contains more details.

The title "Parachain bootstrap waits up to 67s for relay finalization" also sounds like this is a surprise, but by design we normally wait for warp sync to finish.