(In reply to Clemens Eisserer from comment #5)
What are "modern intel desktop GPUs"?
Right now that means Skylake and newer. Very shortly it will be Haswell and newer.
I had a not-that-great experience running WR enabled on Intel Skylake (HD 520) on both - Windows and Linux on a 4K display.
Rendering was pixel-perfect, performance however was not that gread - Firefox felt somehow sluggish on many pages.
So no hard numbers, just a "feel" - maybe pushing 4k pixels at each redraw is a bit tough for those GPUs (this makes me wonder how WR will perform efficiency-wise on battery-powered devices - memory bandwidth is quite costly energy-wise).
Is your experience recent? i.e have you tried since picture caching was enabled? If so can file bugs for any pages you encounter this today?