Flutter re-generates the entire layout every tick and diffs it (immediate-mode), like a game engine. If your device isn't quite fast enough it'll lag, yep. RN is retained mode (but written in immediate-mode style and the diffing only happens when it has to).

Interesting, I didn’t know that it was immediate mode. According to the article [1] though, it now uses rendering engine with Retained Mode due to performance issues.

[1] https://medium.com/@0s.and.1s/flutter-part-iv-skia-vs-impell...