M566 X150 Y150 Z12 E9000 P1
Those values are very very low, and if you were trying to go lower, you're not likely to see any difference.
The units for M566 is mm/min and the lower limit is 0.1mm/s or 6mm/min.
So 150mm/min = 2.5mm/s.
Are you also setting M205 anywhere?
I would start with
M566 X600 Y600 Z60 E6000 P1
Thanks for the reply. I've tried all sorts of values with no visible impact on the print speed etc. I've even gone down to the minimum allowed values of X/Y6 with no effect. I am not aware I am setting M205 anywhere - it's not in the gcode files I am generating.