I solved the problem now and V1.16 playes videos as it should. The reason was not Wayfarer.
At moment I see no reason to downgrade. I mostly use G5 or G4 (1.67GHz) which have enough power to see YT "stutter-free" with astonishing low CPU-load - even on PEG2!. So I'm a bit surprised, that it does'nt do well on a Mini with a 1.4 GHz G4!
I also cannot really confirm that V1.12 was quicker - ok, I've no V1.12 left to compare now and maybe I don't remember right.....