Interesting fact: the 50ms to 100ms grace period only works at the very beginning of a user interaction. You get that grace period when the user clicks a button, but when they're typing in text, continually scrolling, clicking to interrupt an animation, or moving the mouse to trigger a hover event, it's better to provide a next-frame response.

This means that it's safe for background work to block a web browser's main thread for up to 50ms, as long as you use CSS for all of your animations and hover effects, and stop launching new background tasks while the user is interacting with the document. https://web.dev/articles/optimize-long-tasks

I think under 400ms is fast enough for loading a new page or dialog. For loading search suggestions or opening a date picker or similar, probably not.