Thanks, but it seems it came from Windows Defender : there was a new update and now everything is back to normal.
Really seems like it's the previous update that made a mess...
Got the same problem here since the updates of March 12th : when I start the computer, it runs fine, but after about 15 min, it starts to lag a lot since the system interrupt usage spikes a lot when I move the mouse (about 40-50%), but if I unplug, it the system interrupt usage goes back to...