I've already spent a long time trying to optimise the event sheet layout. Unfortunately though the layout is all calculated by the browser, and improving layout performance in browsers seems to be a kind of dark art that is barely covered by any documentation or official performance advice. You can find some blog posts on the subject but they cover just the most simple, trivial changes that we're long past. So I don't really know what else could improve it. Maybe different browsers perform better.
In short the more content that is visible (i.e. expanded) in the event sheet view, the more layout work the browser has to do. It's possible some combination of CSS properties would improve it, but we have little more than sheer guesswork to go on, and I already spent a long time on it. You can play around with it yourself tweaking styles in dev tools. If you find anything that makes a significant difference, I'd be interested to know what that is.