Everybody seems obsessed with circles.
Now are you cutting a circle using G2 or G3 command or have you got a cad and cam program that make your cicle up from lots and lots of little lines.
G2 and G3 are catered for on Mach, and smoothly alter the acceleration and decelation of the axis is sequence.
With lots and little straight lines, Mach has to stop after each line, calculate the next line and carry on.
You can smooth this out by selecting CV mode in config, but cutting the circle in G2 or G3 is preferable.