Hello, i have low fps in the game and low res textures as well despite the fact that i have a good rig. Any idea ?

Yea, It's 30 fps locked for me. Does anyone know how to change fps? And does Hi-Res Texture Pack (Steam version) help to get better textures/graphics?

The low FPS is likely a v-sync issue. I had this problem. Try turning off v-sync. There is no in game setting for this, so will have to do it using GPU control panel or edit the games settings file. You will likely need to cap the fps to 60 as well.
Hope that helps.

Go to Documents\My Games\Fallout4 and open Fallout4Prefs.ini.
Then make sure those two lines look like this:
bBorderless=0
bFull Screen=1
It removed 30fps cap for me and set it on 60fps.