The CA path does not handle large numbers of layers well. IIRC, it has to do with needing to write changes to a CATransaction, which has a fixed overhead per layer. We never came up with a solution to deal with this. I wonder if this is a fundamental CoreAnimation limitation, or if there are workarounds we're not aware of.

Hmm, I added some counters to see how many layers we're updating, in particular
- content updates
- contents rect updates
- transform updates

Quick sample:
  Contents: 379, Rect: 413, Transform: 472
  Contents: 366, Rect: 412, Transform: 493
  Contents: 418, Rect: 447, Transform: 495
  Contents: 401, Rect: 435, Transform: 491
  Contents: 277, Rect: 349, Transform: 484
  Contents:  24, Rect: 150, Transform: 482
  Contents: 442, Rect: 479, Transform: 488
  Contents:   8, Rect: 114, Transform: 486
  Contents:   0, Rect: 111, Transform: 484
  Contents: 241, Rect: 316, Transform: 488
  Contents: 135, Rect: 202, Transform: 489
  Contents: 356, Rect: 380, Transform: 488
  Contents: 222, Rect: 287, Transform: 489
  Contents: 155, Rect: 225, Transform: 489
  Contents: 245, Rect: 296, Transform: 488

So we're failing to match the DOM layers to CALayers and we end up doing way more work than we should...

Not sure if it's useful, but I also see this effect on Google Docs (a single page document):

Deleted: Screen Shot 2018-04-27 at 10.03.15 AM.png 191 KB

	Screen Shot 2018-04-27 at 10.03.15 AM.png 191 KB View Download