Recently we determined that OWC RAM modules that are compatible with the August 2020 iMac (iMac20,1 and iMac 20,2), will report and operate at a lower speed (2133 MHz instead of 2666 MHz) when paired with modules from other manufacturers, including Apple OEM RAM. This occurs on all versions of these iMacs (i5, i7, and i9), and between modules from other manufacturers as well.
The best way to avoid down-clocking is to install only Apple or only OWC modules which are identically specced. Here is the statement from Apple's support page relating to this issue: "For optimal memory performance, DIMMs should be the same capacity, speed, and vendor. Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:
- 2666MHz DDR4 SDRAM"