I did some testing on this a while back.... up to about 400 fonts.. no problem... over 600... slowdowns begin.

This is a Windows thing, really. Fusion is just more sensitive to it than you would expect.

Font managers were created because of this sort of thing. Not Fusion-related.

It takes a while for the OS to generate a dropdown to choose a font from when there are a huge number. System speed also being a factor, of course... but with todays computers, not so much.