I'm using one of the few years older version of Mach4 Hobby and there is no such feature in it (spindle speed variation). I don't know if the latest has. But is it possible to add this feature with some LUA scripting?

My goal is to reduce chatter.

I've managed to add SSV to the machine. Here's a video I've uploaded.

https://www.youtube.com/watch?v=RPLbcykSZIM