Extended length of roughing toolpast with 3mm EM

Hi,

I'm a relatively new user, and am practicing milling double-sided, with a work area of 240 x 200 mm. Each side has two toolpaths, one "rough" with a 3mm EM, the other a finishing cut with a 2mm EM. I'm generating the toolpaths with VCarve Pro, to "Mach 2/3 Arcs (mm) .txt", then running these sequentially on an 840/2. The 3mm EM is configured with 4.1mm pass-depth, 0.6mm stepover, 1800mm/sec federate, as per attached screenshot.

Here's the problem:

Vectric Pro times the roughing pass at circa 35 minutes, at a feedrate of 1500mm/sec, but the elapsed actual time for the 3 passes is nearer 6 hours! For the life of me I can't spot the error in the job-set-up. Any help in running this problem to earth would be most welcome.