This is disappointing. I bought a PS5 Pro mostly for this game. I didn’t want to make the trade off between graphics and frame rate, which seemed to be what the base PS5 often makes people do (from what I read).
I guess we can hope for a future update once they figure out how to optimize more.
If you have a jailbroken PS5, you can get FPS patches that unlock the framerate for many games. RDR2 runs smoothly at 60FPS in 1080p with the patch [1]. I suspect that such patches will be released for GTA6 as well soon after release
1. https://youtu.be/QACSZ2S5HLo?t=232