There really are only a handful of things you could do that should passably take "hundreds of milliseconds" and loading a web page is not among them. Neither is displaying a few hundred thousand things on a screen, least of all what are essentially rects in 2D.
The only reason any of the above sounds fine to take this long is because the standards of web development are so incredibly low.
sure if you have the access directly to drawing those rects in 2D, but if you are using a library to do drawing of the rects you may have a small tradeoff of time (obviously not 100s of milliseconds) and if you are using a library that abstracts away writing to the DOM for you and then the browser takes the DOM and essentially figures out how to draw a hundred thousand things on the screen you might get in the hundred milliseconds area - depending on a whole bunch of things that are varying from time to time.
The only reason that 100s of milliseconds might sound fine is because there are multiple layers of abstraction between levels, but as you can see from all the other responses here the 100s of milliseconds evidently does not sound fine.
The only reason any of the above sounds fine to take this long is because the standards of web development are so incredibly low.