iMac runs really slow

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.

Your problem is simple, you have never upgraded the RAM and then you are strangling the machine by adding AVG antivirus which at the very best is 100% worthless. Mac OS does not benefit from or require in ANY WAY any third party antivirus, cleaning or any other third party "maintenance" apps. If simply kept up-to-date and otherwise left alone Mac OS will remain secure and stable. If you want a slow, unstable and buggy system then please keep AVG installed. Please locate AVG's uninstall instructions and then uninstall, next restart the computer in Safe Mode and then retest. Use safe mode to isolate issues with your Mac

When upgrading the RAM, please upgrade to a minimum of 16GB and please only use Crucial.com or Macsales.com RAM.

1) Your iMac is running low on RAM.

You can easily and inexpensively add a pair of 4GB or 8GB modules into the empty slots and have 12GB or 20GB.

For a RAM upgrade, see > https://eshop.macsales.com/shop/memory/iMac/2010/DDR3_21.5_27

the 8GB (4GB x 2) kit is only $47.88 and the 16GB (8GB x2) kit is only $84.88

2) Your 500GB HDD looks OK, but is under preforming, either from age or from the use of AVG and MacKeeper.

The only way you could increase the general speed is either to upgrade the internal hard drive to an SDD or get an external Thunderbolt SSD drive. However, the external Thunderbolt drives are very expensive.

If your drive is going bad I'd seriously consider getting a new 27" iMac, one with an SSD drive and at least 8 GB of RAM, preferably 16 GB. Pouring money into a vintage 9 year old Mac is not cost effective. I had a 2011 iMac go belly up a while back and didn't try to resuscitate it. Not worth it.

You will need to locate an Apple Authorized Service Provider in your area and make an appointment. Ask them if they can replace the HD with a SSD. This will dramatically improve performance and also should be more reliable. Begin by clicking the Support icon in the upper right of this screen.

Have a look at > Install memory in an iMac - Apple Support

Plus there is an install video at > https://eshop.macsales.com/shop/memory/iMac/2010/DDR3_21.5_27