To be fair shadow of the Colossus has pretty bad frame rates even on the original hardware, averaging 15-20 fps. So I'd say if anything that's an accurate recreation of the original experience :)
> Well, SOTC is a game whose frame rate can increase and decrease
> wildly. Actually, we have incorporate the variable frame rate
> in the design - it increases and decreases with load balancing.
> Although there are cases when it reaches 60 fps, there are also
> times when it falls to the 15 fps range, but the motion blur helps
> to smooth over this, and the player's sensation of frame rate
> changes is held down to a minimum.
Unlike the PS3 remaster - the PS4 remake is built on a completely different engine and doesn't share any assets (except maybe some audio) with the original.