My iMAC is sooo slow what can I do?

close some backgroud programs not used at the moment from the sidebar?

Most issues with Yosemite are caused by third party software like “cleaning”, “anti-virus” or memory-“management” applications. Do you have any of these installed?

Please post an EtreCheck report: that can reveal possible causes of the slowness.

The OP is running Lion, so there's nothing associated with Yosemite.😕

27" i7 iMac (Mid 2011) refurb, OS X Yo (10.10.1), Mavs, ML & SL, G4 450 MP w/10.5 & 9.2.2

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

I suspect that you clicked the wrong posting, but glad you found enough info to fix the slowdown.