It probably is voltage drop if thats all you have done and sure you haven't messed up the wires. Thou the Speed reference voltage must have been on the low side to start with for only 1mtr to affect it that much.
Often the Voltage source is 10V but using a Pot etc you never actually get 10v which the speed controller needs for full rpm.

Start by measuring the voltage at full speed and see what you get. You may need a higher voltage source like 12V to get full 10V for the speed controller.