Yes I think you are right, I can see it peaking sometimes and rarely up at 50%, but generally for woking around 10-20%, 4% memory.
So was looking a bit further and think its the video rendering. Today the machine (being my laptop and undocked) at 1366x768 res seems to be better than when its plugged to external VGA (1920 res). It especially noticable when you select edit text (ie a large redraw), I have typed 3 4 chars before the box even can appear and misses those chars.
Click the edit tool today and I can count to 2sec before the new toolbar is there.
Messed around with Libre office and thats quick, so do not think its machine. Video acceleration is working fine, glxgears etc.
Its not a big deal, but it does seem less responsive than Qcad2.2, you have to set your mind to pause between each click and generally feels less productive.
Switch away from QCAD > Another App > QCAD > choose Edit tool = 6sec
Stay in Qcad choose edit tool =- 2-3sec, backup, select Edit tool again and its still 2-3 sec.