There are two ways of running - Constant Velocity and Absolute Stop - see on the Config page.
In absolute stop, the cutting axis will come to an absolute stop BEFORE the other axis starts to move - this will produce a "square" corner - althoug cutting internally you do, of course loose the tool radius - which you can file out if you wish.
In Constant velocity - Mach3 reads forward and anticipates the next command, and as one axis is decelerating, the other axis accelerates so the tools ic constantly cutting - but this leads to a rounding of the corners - and the faster you go the "worse" it gets, becasue the axis need more time to accelrate/decelerate.
As Hood stys there is the ability to alter the parameters of Constant Velocity to make the acceleration/deceleration quicker, or only active between certain distances,
On a right angle cut as you are doing, absolute stop may not be too much of a distraction. Where you get an arc to cut, which a cad program makes up of many small straight lines, then Mach3 still stops between each one - producing a very jerky movement - hence the need for constant velocity.