It happens when you set your rMBP display to "Scaled" in display preferences, which is what most developers do. (Edit to your edit: Try the "More Space" option in your display preferences! It's amazing.)

Similar to the sibling comment, I can't seem to repro. I switched "More Space", restarted Firefox, loaded up a dozen or so tabs. I even watched a YouTube video for about 10 minutes. The video did cause CPU to jump a bit, but it dropped when I unloaded the tab, and nothing I've done (including the video) has caused any sort of lag like described in your first post or subsequent follow-up.

I'm not sure what to say. I'm sure the issue is real, but it seems like we're missing something in the triage. There has to be something else that makes a difference.

I tried this as well, can't reproduce.

I've got a 15" "late 2013" rMBP, set display options to "more space." I've got a total of 45 tabs open between a couple different windows (I use the userChrome hack to re-enable multiple tab rows.) Cycled between tabs, refreshed, navigated around FB, GMail, etc. Can't get the CPU to do anything out of the ordinary.

I very much doubt that “most” developers use “More space”.

Most developers in their early 20s seems plausible, but even there, I surveyed my team’s younger cohort, and it was less than 50%.

I have my display set to "scaled". I get the worst slowdown when streaming videos. 2013 15" w/ Nvidia gpu.