Hi, Chrispol
Try setting your Accel UP some, Between lines it's slowing to 9 every time.
If "F" speed is below your accel speed it takes priority.
I usually set the accel up to a point that steppers will slip or servos would trip, From a stopped condition (well maybe not that fast for some servos thy get pertty scary).
Then take the value and reduce it some for no slip/lost steps, 900 to 9 is pretty large.
Use G00 Moves to test these speeds, May need to back off your 900 max. value after testing.
Hope this Helps, Chip