Technically, everything causes a performance loss You could load the chunk into an async task and check the blocks there, but I think serialising is slower than just checking 4 blocks lol I don't think there is an easier way, however I'm far from an expert in the PMMP field.
It would be a one block check if you want to find the chest a sign is attached to... https://github.com/pmmp/PocketMine-...94bd024/src/pocketmine/block/WallSign.php#L35