Therein lies the problem - the acceleration is just far too low. You want at around 500-1000mm/s^2 on a CNC router, so re-tune your motors to obtain that. You will probably have to reduce the rapid feedrate a little.
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Whoops my bad sorry got them mixed up. That is the max velocity I mentioned above. The acceleration on all axes is above 700mm/s^2
I've downloaded mach3 trial and am going to give it a try.