So to sit back and say "yeah there's nothing we can do about it, web apps are slow" is just wilful ignorance at best and learned helplessness at worst.
Easy to do fun things instead of tech debt; we didn't want to optimize NodeBB back then either, we just had a client breathing down our necks to fix it and fix it fast.
But Mastodon is not NodeBB from 10 years ago. This software is used by 1M times the amount of people that used NodeBB. Is that not motivation enough?