Adding Ram to 2020 27 inch 5K IMac with AMD Radeon Pro 5700 XT 16 GB

I purchased two OWC 16GB 2666 Mhz cards and installed them in slots 2 and 4 leaving the two 4GB Apple cards in slots 1 and 3 in my 2020 27 inch 5K IMac. The computer is running fine although the fan is now clearly audible when it cuts in whereas I had never heard it before. However, my Mac now shows the memory as 2133 Mhz whereas it was showing 2667 prior to the new cards being added. I have since heard that this drop in speed can be rectified by moving the Apple Card from slot 3 to slot 2 and putting the new cards in slots 3 and 4. The only problem with that or so I have heard is that with that configuration it would not be running as dual core. So, if this is correct, would it be better to have 40 GB Ram dual core with 2133 Mhz or single core with 2666Mhz?