[Chrome] Actions outside of a large notebook are sometimes are slow

## Description

After a large notebook fully renders, clicking on menus, tabs etc. are all much much slower at times (i.e. this isn't always slow on Chrome). I have noticed this in Chrome moreso than Firefox.

The way it seems to be reacting is as if the whole main thread is blocked. Some profiling showed large "Recalculating styles" but not much more to go on.

## Reproduce

Use [generated-1000cells-0.9ratio-2loc.ipynb](https://github.com/jupyterlab/benchmarks/blob/master/examples/from-script/generated-1000cells-0.9ratio-2loc.ipynb) and let it fully load. Click on a menu (e.g. File, Kernel) or switch between the file browser and ToC. The video below took a couple of tries and shows that it took a few seconds at times to respond to a mouse click. I have seen it much, much worse than this, but have not yet been able to create a reproducer.

https://user-images.githubusercontent.com/1813603/155785679-a1902ca9-0333-4487-bdec-a110871275ad.mov

## Expected behavior

It's fast!!

## Context

Lab: 3.3.0rc0
Chrome: 98.0.4758.109
Firefox: 97.0

Start with examples/from-script/generated-1000cells-0.9ratio-2loc.ipynb and let it fully render in Chrome 98. Profile menu, tab, and file-browser/ToC interactions, focusing on the reported large “Recalculating styles” periods. Done means the same large notebook no longer causes multi-second interaction delays in Chrome.

Written by the indexing model from the issue text.