A couple of left field thoughts...
Have you checked the temperature of your processor ? Plenty of widgets around to do that. I read an article that says OS X manages overheating by removing access to the processor...
https://discussions.apple.com/message/16634745#16634745#16634745
May late 2011 macbook can go into full blast fan whenever I watch videos, or a browser has some flash advert running in the background....
With any old laptop dust collects in the fan creating a felt mat that stops airflow which can cause the heat build up - I have to take my daughters machine to bits every 6 months or so to clean it out.
The other thought is Spotlight indexing - with a new machine and restored backup you probably just need to leave the machine alone for a day or so to let it index the drive and not get in the way....
cheers
Bob.C