There are already several topics on Kirby performance with lots of pages, for example: Kirby performance with over 60K simple pages
I think you should really do a test yourself, it’s not difficult to set up a test site with some fake data. As long as you keep a tree structure, use a VPS with SSD instead of shared hosting, I don’t think you will run into problems. You could even add a SQL-Lite database for indexing.
@bnomei, for example, claims he has created a Kirby site with 3000+ pages