Ninlopthis would be down to gpu and cpu limitations, AFAIK if your pc is cpu limited then you'll find that graphics settings will have little to no affect on your frame rate but lowering your resolution will increase frame rates, and if your pc is gpu limited then you'll find that resolution has little to no effect on frame rates but lowering graphics settings will increase frame rates
most people won't be in one extreme they'll likely be slightly gpu limited or slightly cpu limited, so its a safe bet to run on lowest settings and youll most likely see an improvement
Resolution is more intensive on the GPU than CPU. Things like prop physics, ragdolls, some lighting calculations, sound, etc are affected by CPU performance.