The issue of Mantic seeming to "load" the gpu quite a bit on cold boot has recurred several times since moving to Mantic. It seems to run it at higher "revs" for 4 or 5 mins before dropping itself down to "normal" speeds . . . this is before even launching FF . . . i.e., not asking the system to do anything except boot itself up, etc.
This was not an issue in the previous iteration of Lubuntu, but seems to be a "Mantic" thing?? The gpu is an upgraded Nvidia 780 I believe . . . running nouveau.