TWO important points:
- Corsair RAM has not played well with Intel-powerd Macs. They have two tiers of RAM quality but, due to the price-sensitive nature of the aftermarket RAM business, few resellers carry the higher-priced version that seems it be OK in Macs. For me, it's Crucial or OWC, or nothing.
- Why are you adding RAM? If you expect the computer to run faster, 12 GB RAM is not the bottleneck. We have the same model iMac with only 8GB RAM runing High Sierra 10.14, and diagnostics show it is never starved for RAM.
If your iMac is slow, it is the 11-year old, old-school mechanical hard drive. Ours has the entey-level SATA 3G 7200prm drive, which is still faster than the base storage in later 21.5-inch iMacs. (SATA 3G 5400rpm). Still, today the 100MB/sec write/read speed of the 2011 drive feels sluggish. Ours take a long time to boot and big apps like Office take long to launch.
So before spending money on more RAM for and old Mac, recosider what you expect it to accomplish. If the old HD is slowing you down, all the RAM in the world won't change that,.
Only an internal solid-state drive upgrade can help you 2011 model, and that is a complicated upgrade. NOTE: The oft-mentioned external USB SSD option does not apply to pre-2012 iMacs because they have USB2 ports. The external option might even be slower thatn what you now see, and a complete waste of money.