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.