On Nightly/OSX 39.0a1 (2015-03-16) If I let the display sleep (but not lock) and then wake it up again, Nightly becomes almost completely unresponsive. Menus continue to be actionable, but tabs and in-browser events take 30+ seconds to register or display changes. Nothing out of the ordinary shows up in the console when this happens, but the browser is completely off in the weeds. The effect goes away after about two minutes, and things return to normal snappiness. I can reliably duplicate this.
I've seen this as well on Mavericks.
Also, it felt to me like painting was just broken vs CPU usage, etc. Maybe vsync related changes? Has that landed in Mac?
Should be fixed in today's nightly...
Or rather tomorrow's apparently.