First of all, thanks for the help guys :)
So, I did the 720p test in 3 games:
RDR2 was around 50 fps on 1080p, and become around 68 fps at 720p
Assassin's Creed Odyssey went from 52 fps at 1080p to 58 fps at 720p
I also tried with Resident Evil 3, and strangely in that game, I had more fps...