If you have an FPS or interactivity target that needs to be guaranteed to be better than about 0.1-1 second - depending on platform - you can't have any disk IO at all on the UI thread, or any thread that's supposed to react at interactive rates at all times.

It has generally become much better in the last decade or two, but one should still expect most OS's to sometimes pause for excessive amounts of time on disk IO unless the API is specifically guaranteed to never pause. Even then one would be wise to measure/log deviations if it's critical for the application. OS guarantees might also be contingent on driver/subsystem guarantees, and bad drivers might sometimes affect what seems completely unrelated upstream systems.