There are a couple of things you need to do to speed up the printer:
1. Put commands in config.g to increase the maximum allowed movement speeds, and possibly the accelerations too. I use this:
M201 X800 Y800 Z10 E350 ; set accelerations, increase E
M203 X12000 Y12000 Z180 E3000 ; increase max X, Y and extruder speeds
2. Change the slic3r parameters to use higher speeds. I have left the perimeter speed at 20mm/sec but increased solid infill to 40mm/sec, other infill to 60mm/sec, and travel to 200mm/sec.
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].