Developer Menu -> Throttle FPS If Not Focus is checked. Deselect Interface Window - FPS remains at full rare same as selected, fans screaming on vid card etc. Issue is a fight between option to throttle fps when inactive and a choice of setting fixed framerate in Display -> Framerate
If you select anything other than unlimited there Interface forgets to throttle when not active window. No big deal, but it does indicate a bit of messiness in logic that could bite in nether regions at some point.