Den.thed's point #3 is absolutely on target. The hard drive is struggling. I like lots of RAM but, given the drive's current performance report, adding RAM now will not create a blistering speed increase. We have the same 2011 model with the same hard drive as yours:
disk0 - WDC WD5000AAKS-402AA0 500.11 GB (Mechanical - 7200 RPM)
Internal SATA 3 Gigabit Serial ATA
Your drive posted these scores:
Performance:
System Load: 1.24 (1 min ago) 1.64 (5 min ago) 1.77 (15 min ago)
Nominal I/O speed: 0.47 MB/s
File system: 75.22 seconds
Write speed: 37 MB/s
Read speed: 50 MB/s
Ours, without any antivirus dungware, recently posted these scores in the same test:
Performance:
System Load: 1.62 (1 min ago) 1.67 (5 min ago) 1.87 (15 min ago)
Nominal I/O speed: 0.05 MB/s
File system: 38.28 seconds
Write speed: 112 MB/s
Read speed: 91 MB/s
So something is seriously wrong with your drive. Removing the anti-virus stuff may help but the drive itself remains my prime suspect.
For a 2012 or later iMac with USB3 ports, the easy solution is to use as USB3 external enclosure containing a fast (6GB/s) solid state drive as your boot volume.
Unfortunately that option is not available to the 2011 and earlier iMacs because they lack USB3 ports. Trying the external solution will actually slow the computer far below its current underperforming state.
For 2011 iMacs the option that remains is to replace the internal mechanical hard drive with a 6GB/sec solid state drive (that matches your hard drive bus speed). Based on other Macs to which I have applied this solution, your read/write speed will jump the the 500Mb/s range, 10X what yours reports now That will make an amazing performance difference.