IPHONE 12 Prox Max not charging rapidly anymore. I'm using 20W Power Adapter. What is the problem?

Good. Then suggest the following troubleshooting plan:

(it might take a while - i.e. days - to get thru all the possible permutations)

First turn off “Battery Optimization”

Then make sure that the phone’s charging port is free of any lint or other debris. Verify that all the gold contacts in the port appear uniform and not deformed.

Then perform a complete power-off reset of the phone.

If no change, try changing power adaptors AND cables.

Post back your findings.

You understand that the fastest charging is up to 50%? It then slows a little up to 80%, and the last 20% is very slow to avoid overcharging. So it should reach 50% in 30 minutes, 80% in another 15 minutes, but can then take an hour or two before it reaches 100%, partly because it pauses at 80% to cool down.

It could possibly be caused by a non-compatible cable.

Are you using the originally supplied cable; or at least a cable which previously fast-charged?