@Justin: the performance fix indeed works and the project is way smoother for me. But I found out why I couldn't see the improvement in the first place:
when a project has folder tracks collapsed, the scrolling and zooming is still slow.
collapsed:
https://i.imgur.com/bQJEJwa.gif
uncollapsed:
https://i.imgur.com/xdd3BWQ.gif
EDIT: btw, that project has 1243 tracks.