We were running into this issue after recent changes to our CSS used by _repr_html_ to display our objects. The problem got worse with the number of cells in a notebook, even if those cells were empty.
It turned out that putting CSS custom properties into :root was the cause (see Do not put things into :root to avoid bad performance of _repr_html_ by SimonHeybrock · Pull Request #1847 · scipp/scipp · GitHub). Placing the properties in a specific class brings back the original performance.
Note that this does not solve all problems: I still experience slow UI once about 100 or more cells are in a notebook, but it is not as bad as when CSS :root was modified.