Question: Slow Mac.
I downloaded Yosimite about 12 months ago only to find my system hobbled with the rainbow whirly thing. Desperate I followed advice on this site and re-fomatted the HD and re-installed Yosimite. It took me nearly all day. All fine now and MacBook is performing well. I think the original download from Apple was not integrated properly and probably I had a half and half installation.