Smoldot freezes browser UI on sync finish

[tuul-wq]

In our SPA we use @polkadot/rpc-provider^9.4.2 to connect 3 ScProviders
During the connection UI works fine, but just before sync finished message in console we face a 1.2 second freeze.
This happens every time when sync finished for 3 chains that we connect with ScProvider - Polkadot, Kusama, Westend.
We're not sending any requests to ScProvider, it just works by itself.
I recorded performance check in Chrome and saw a long wasm function execution

(func $func5418 (param $var0 i32) (param $var1 i32)
    (local $var2 i32) (local $var3 i32) (local $var4 i32) (local $var5 i64)

Log message example: [sync-service-polkadot] GrandPa warp sync finished to #12229848 (0xea75…6e7b)
smoldot version 0.6.34

[tomaka]

This is most likely the time it takes to verify the finality proofs.

While there's a "CPU limiter" in place, I suspect that it doesn't work because the inner FuturesUnordered immediately re-polls futures that are woken up during their execution.

[tomaka]

cc rust-lang/futures-rs#2053

[tuul-wq]

@tomaka thanks for the fix, can't wait to see it merged and released ☺️