Late 2012 Imac 27'' constant beach ball loading and slow downs
It might help to examine how the machine's resource usage looks inside of Activity Monitor...Are the CPU cores maxed out? How about RAM usage - is there any green showing in the pie chart? Lastly - is the Disk Activity tab showing excessive write rates constantly occurring? 

Does the performance of the machine increase when you quit out of particular applications (namely Parallels?)

A defective hard drive could cause slowness, but this may be accompanied by other symptoms (crashing, audible clicking, logging of I/O errors). It may be prudent to run the Apple Hardware Test just to confirm tht the hardware is in good shape.