El Capitan issues

[CeltiadLad]

Has anyone got any thoughts on why my iMac keeps freezing for 1-2 minutes at a time after installing El Capitan 10.11.1?
Screen and or mouse freezes constantly, have tons of memory available.

 

[ffg7]

was it doing that before as I just finished installing that os on a customer's imac that had a bad hard drive & does not have issue now on new hard drive?