It's not related to tree-sitter, but recent work on using text properties instead of overlays for folded regions in org has improved performance opening org files with folded regions from O(n^2) to O(nlogn). See https://blog.tecosaur.com/tmio/2022-05-31-folding.html It's a big improvement in practice.

Sounds promising, thanks. The most annoyingly slow thing is the agenda buffer which takes ages to build. (Have to admit that the files from which it is sourcing entries are humongous after 15 years of managing my life with org mode.)