iMac 2020 3.6 GHz 10-Core Intel Core i9 running slowly since RAM upgrade

Even when purchasing RAM from those recommended vendors, you must also make sure to only purchase the exact part numbers those sites recommend as compatible by using the tools on their respective sites to identify compatible memory. It is not safe to just purchase memory based on memory speed these days since these computers can be very picky with the other memory timings not normally shown by Apple or even sometimes the memory vendors themselves.

You do want to make sure the new RAM is fully seated in the slots and the memory mechanism has been fully latched into place (both sides).

You can also remove the new RAM & reinstall the original memory into those same slots to see if performance has returned. This also helps you to know that you have fully seated & latched the memory in place.

Where did you get the RAM modules?

What brand are they?

Try removing them and putting the Apple RAM back in and check the performance.

Also have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?

deane4work wrote:

I just can't find the option to delve the thread answered or closed and I've looked everywhere! Have they changed something?

I believe marking a post with "Best Answer" is equivalent to the traditional "Solved" designation. Apple changed the behavior of the forum a year or so ago.

Below your last post should be a "Best Answer" button.

Since it was poorly seated modles that you fixed mark your last post as Best Answer.

What brand of RAM? iMacs can be quite ticky with respect to brand of memory.

Other than switching the RAM cards to the current empty slots, then an issue of RAM quality comes into play. I never purchase any other RAM than from Crucial or Other World Computing.