I got a K40 laser with C3D running GRBL-LPC.
With laser engraving and cut rate set to 250mm/sec the laser stops for a fraction of the second many times on the engraving part.
Where laser just moves but does not cut it moves smoothly without stopping.
I an running LW4, latest version 991.
I am runing it on Win 7 64 bit on 2 core processor.
Even if I slow down speed to 150mm/sec it is the same.