Yosemite is sooooooooo slowwwwwwww, the crashes reminds me of windows in 1980's

See:

How to eliminate slow Mac problems,
Mac Maintenance Quick Assist,
Mac OS X speed FAQ,
Speeding up Macs,
How to Speed up Macs,
Macintosh OS X Routine Maintenance,
Mac troubleshooting: What to do when your computer is too slow,
Essential Mac Maintenance: Get set up,
Essential Mac Maintenance: Rev up your routines,
Maintaining OS X,
Five Mac maintenance myths, and
Myths of required versus not required maintenance for Mac OS X for information.

27" i7 iMac (Mid 2011) refurb, OS X Yo (10.10), Mavs, ML & SL, G4 450 MP w/10.5 & 9.2.2

Please describe the problem in as much relevant detail as possible. The better your description, the better the chance of a solution.

For example, if the computer is slow, which specific actions are slow? Is it slow all the time, or only sometimes? When did it start to be slow? Did you change anything just before it started? Have you seen any alerts or error messages? Have you done anything to try to fix it? Most importantly, do you have a current backup of all data? If the answer to the last question is "no," back up now. Ask if you need guidance. Do nothing else until you have a backup.