Frontend & browser·JavaScript in the browser
your loop over ten thousand rows froze the whole tab, including the scroll and the spinner.
Main thread
Also calledUI thread
The single thread on which the browser parses, runs your JavaScript, computes style and layout, and paints. Because all of that is serialised, any script that runs for long blocks everything else, including the animation you added to reassure the user. This is the constraint every other performance term on this page is downstream of.