Friday Night Video CXXXVII

I have a iMac.  I updated to the latest OSX release, Yosemite.  My iMac now runs like shit.  Currently moving all data to an external drive in preparation for wiping the main drive and reinstalling from scratch, which may help.  If it does not help, I will be quite vexed.  I suspect it may help, as a friend assures me he has an iMac which is older and has less RAM than mine, and Yosemite runs fine.