I recently changed to AC12 from 11 and have also noticed not so much that it is slower but that it "pauses" mostly after editing Library objects and labels but also other occasions to.
Bring back ArchiCAD 6.5 it was quick
AC versions 3.41 to 25 (UKI Full 5005).
Using AC25 5005 UKI FULL
Mac OSX 10.15.7 (19G2021) Mac Pro-2013 32gbRam AMD FirePro D500 3072 MB graphics