Now, this is also all empirical, since I haven't been measuring things until I felt problems.
But...
I was able to play the game smoothly at maxed out RTX without DLSS for tens of hours in last weeks. Then, when Phantom Liberty got activated, my eyes started feeling tired rather qucikly. I brought up the frame counter and that was sure bad... the game was at 40-ish fps. If it is right that Phantom Liberty activation introduces things that drop performance that much, then it's at least a lucky thing they are not affecting performance before PL activation.
Any other observations?
Btw, for me PL activated after rounding up the first of the three big quests (the Voodoo boys part) in search of who will help with the Relic. Like, straight after it I got the call from Song, didn't walk 20 meters from the place where I had the convo with Johnny.