GlyphCollectionView uses lots of CPU

[frankrolf]

I just discussed this with @benkiel, who encouraged me to open this issue.
My setup is Robofont Version 3.3b (build 1905221146) on macOS 10.14.5. I noticed some slow scrolling behavior in the Font Overview, and could isolate the source to be the GlyphCollectionView.

My observation is that any window using the GlyphCollectionView is scrolling very slowly. It’s easy to max out the CPU by just scrolling up and down in a glyph window – both in Robofont and the DefconAppKit Test App. The “rubberband bounce” effect at the bottom and top of the window is not smooth.
Overall it seems there is a large load on the CPU when scrolling – which of course gets worse the bigger the UFO gets.

I don’t know where to start with this, but I’d be glad to help out.