Mach3's trajectory planner is far from perfect.
I've seen hesitations between tangent moves, when there should be none. And I've cut identical parts where it hesitated on one, but not the other.
If it's not a g-code issue, there's probably little that you can do about it.
Mach3 also has no control over the actual path deviation. So when you tell it to go as fast as it can, there's no guarantee how close it will follow the path when it changes directions.
The various controls basically just let you specify a distance from the corner where Mach3 should slow down.