I've found that sometimes when I open Krita, the brushes will still lag even if Instant Preview in the brush settings is turned on. I've found the fix for this is by toggling on and off the Temporarily Save Tweaks To Brush Presets option.
Hi, Joseph! Thank you for your report! I have solved this bug recently. The fix will be available in the next release of Krita 3.1. Please check the latest "Krita 3.1 Release Candidate 1", where the bug should be fixed already :) https://krita.org/en/item/krita-3-1-release-candidate/ If you still can see the bug, please reopen this ticket :)