Excessive CPU usage in the editor when selecting objects on Linux

[CounterPillow]

The editor (both debug and release builds) appears to use an excessive amount of CPU time when selecting an object on Linux. As long as the object is selected, my CPU usage spikes up to 75%, which is equivalent to 3 of my logical cores maxing out.

Here's a video demonstrating the issue: 1.7 MB H.264 MP4

Initially I thought this might be related to QTBUG-45959, but setting QSG_RENDER_LOOP=basic did not help so the two are unlikely to be related, unless Optimus breaks more than just threaded rendering.

Further details about the test system:

• nvidia GT 840M
• nvidia driver version 352.21
• Optimus through official proprietary nvidia driver (no bumblebee etc.)
• qt 5.4.2

EDIT: To clarify since someone pointed this out, I have ksysguard set to divide CPU usage by number of CPUs, because usage numbers above 100% are a ridiculous and indefensible. Thus, 100% is "all cores are fully utilised", not "one core is fully utilised". This is the same way Windows Task Manager does it.

[CounterPillow]

Apparently this is the material preview, which doesn't cache preview images and also renders while the material tab isn't visible.