Some comments on the port
(Tested it with 2400/1267/2400 using old EOS)
You can set it below 60fps, though I don't recommend it since the game breaks a lot, for example, at the start you can't take the rings path if fps is at 30; Sonic falls into the water.
If fps is set to 60 and the overclock isn't enough, the game drops to 30fps at 50% speed (even 20fps if the overclock is really low), I'm guessing this is double buffering.
If I set 59fps or lower, when it drops to 30 it'll run at 100% speed, but like I mentioned before, the game isn't built to run that way and bugs show up (in the pic it got stuck).
The resolution scale seems to only affect the title screen; being able to lower the resolution would help hit 60fps.
I tried lowering other graphics options too, though I'm not sure how to fully disable them (shadows, reflections), but I noticed a slight performance improvement lowering them.
The settings I used:
...
ResolutionScale = 1
Fullscreen = true
VSync = true
TripleBuffering = "Auto"
FPS = 59
ShowFPS = true
MaxFrameLatency = 2
Brightness = 0.5
AntiAliasing = "Off"
TransparencyAntiAliasing = false
AnisotropicFiltering = 4
ShadowResolution = "512"
ReflectionResolution = "Eighth"
RadialBlur = "Off"
CutsceneAspectRatio = "Original"
UIAlignmentMode = "Edge"
