Those are very general conclusions to draw from one test case. The bouncing ball test runs at 60 FPS for me on macOS; most of the time is spent in painting, as expected. Likewise, Stripe scrolls at 60 FPS for me.

I should note that the bouncing ball test is the kind of thing that WebRender is designed to improve—painting-intensive animations—so it's obviously untrue that there's no interest in improving this sort of workload. When running your bouncing ball test in Servo with master WebRender, I observed CPU usage of no higher than 15% (as well as 60 FPS)…

> With the page at stripe.com, I don't see any difference between FF52ESR, FF56.0.2, FF57.0b14 with servo enabled, or with it disabled. On my 2012 Macbook Air with macOS 10.12, I see about 98-108% CPU on each, according to iStat Menus CPU meter. With Safari it's about 35%. That's exactly what I would expect based on past experience.

> I would probably close this as invalid, as it's not something new or specific to Quantum or Servo, or as a duplicate of one of the older bugs, though I'm not sure which.

That likely points as to why there's little movement on this bug. It's title can be interpreted to indicate a Quantum regression, but it's a general issue that's longstanding, so it may be the people that are seeing it are not focused on it (they're likely tracking down and fixing actual regressions, not known problems).

I know that doesn't help your issue, but it may help you locate the relevant bug report and lend your weight there, if you feel so inclined.