I have experienced poor cutting speed with complex curves. The feedrate was not constant, even with G64 command.
After googleing few hours, I found a clue: LinuxCNC needs a tolerance parameter to be able to blend complex curves with true constant feedrate. Now I use “G64 P0.01” which gives 0.01 mm allowed error for the tool path. With this feature I have decrease the machine time from 3 hours down to 40 min for the same job, 1/4 ratio !
Below a video showing my machine cutting POMC at 700 mm/min feedrate.