This is a problem Slite seems to be facing as well. We used both for a while and actually gave their teams a lot of feedback. They were very grateful and communicated well, but moved very slowly and performance decreased quite a bit over time. We ended up focusing most of our planning/note taking/scheduling efforts in Linear instead. Much, much different but it works well enough while remaining extremely speedy and productive. I do miss a lot of what documents had to offer our team. I don't miss fiddling around with slow documents which didn't offer all of what we needed (and might never offer it).
I'm not sure why but most document-based apps like this seem to develop terrible performance. I suspect it's genuinely a difficult problem; I don't think I can 'make Notion in a weekend' by any means, or Slite for that matter. I trust they're doing their best.
I also know these tools suit some teams incredibly well, too.