yep, that was also my suspect from the beginning, I tried to remove it but for some reason or another it was still active.

This afternoon I got a new update to the modified hildon desktop pushed, it looked very suspicious, 2 updates in 2 days so I accepted it, rebooted and ... problem solved !. Back to stable, not overheating system

At least I now have a global version of the firmware, the backup size has gone down from 26 MB to 3 MB and at it feels a lot faster. I also think the idle CPU usage is less than before. So after all, I'm kind of happy I had the problems and I was forced to reflash