[spinning this off from bug 1253182]

STR:

1. Visit https://battles.nodegoat.net/viewer.p/23/385/scenario/1/geo/fullscreen
2. Press the minus (—) button in the widget at the top left corner of the page, until you've reached the smallest size the map can be.
3. Click and drag the map back and forth.

ACTUAL RESULTS:
Jank -- the repaints aren't responsive, the map noticeably has to "catch up" with your cursor after you drag it.

EXPECTED RESULTS:
More responsive dragging.

Here's a profile captured by gregp: https://share.firefox.dev/42KvU84
Looks like we're spending most of our time in Category: Graphics | WebRender display list.