It depends on what graphics features you have enabled and your system. Specifically, it depends on where the bottleneck to performance on your system is.
renderscale should mainly help if you are fill-rate bound on your GPU. If you are not fill-rate bound, you may see little to no change with changes to renderscale. It will not help at all if your bottleneck is your CPU.
Just to check, does using /renderscale have any actual benefit? Cranking it way down doesn't seem to give me any increase in framerate.