You don't need to split into multiple files to make large documents manageable, multiple pages works just fine (pages you're not using aren't loaded). But even still, I have absolutely massive pages with ~100 screens on them that work just fine on this base-tier M2 MBA.

Honestly given the complexity of the screens involved I feel Figma's performance is pretty reasonable. (Now, library publish and update - that's still unreasonably slow IMO)