Obviously it's caused by chunk loading, but the spike is too severe, considering it can happen even if the player is right next to their bed.
Resolved. It just takes a very long time, it seems.
Testing on a private Server with randomTickSpeed at 600 and barely any has oxidized.
I assumed copper oxidized faster because a copper block I had placed in the world oxidized rather quickly with the normal rTS.
It was not modified, so it should be the default value.
This is aided by the fact that the nearby crops are growing, and a copper block that I had placed nearby did oxidize, just not the cut copper slabs.
Resolved, caused by Optifine, doesn't occur in Vanilla
Confirmed for 1.20.1