[jillesvangurp]

Not an issue for me. I have a imac 5K and a retina mac book pro. Both with image scaling on by default. I think most recent macs have retina now.

I use Firefox exclusively and have done so for the last 2 years or so. This sounds like it could be a driver issue for specific macs. Both my macs have AMD Radeon chipsets & quad core i7s.

Anyway, I'm sure this issue is real. Bugzilla ticket numbers probably exist for it and might be more helpful than vague complaints about things being slow.

Of course some web sites are a bit unreasonably javascript heavy these days. The downside of a large screen is that pushing a lot of pixels around is not free. Usually closing any offending tabs immediately restores any cpu usage I see. I'd suggest using the new task manager thingy (page menu->more->task manager)

[cpmsmith]

Anecdotal, but I've found it happens when image scaling by a factor other than 2. Affects my setup of a 4K display at 2650x1440 effective resolution, or 1.5x scaling.

[rosser]

Exactly this. I run at 1920x1080 effective resolution on my 15" rMBP, and 1680x945 on my 13". I see the issue on both.

When I switch to the "default" scaling (1440x900 and 1280x800, respectively), it stops. Only the 15" has a discrete GPU, eliminating that as potentially causal.

The "problem" scaling is 1.5x. The default is 2x.