First you want to understand the speed control principles.
Motor speed follows voltage. Look at the voltage applied
to the motor and the voltage sourced at the battery. If
they are equal, in-the-moment (like, don't measure the
motor under load and the battery open-circuit) then the
only help for you is more voltage (and circuitry that can
deal & deliver).
If the output is PWM controlled then the switch FETs
may be always-on at the top end. However if there
remains a minimum off time in the duty cycle, you may
be able to squeeze more out of it by removing whatever
is the duty-cycle bind.
If it's a linear controller (which you'd think unlikely
since you mention MOSFETs) the driver could cost too
much headroom and replacing it with better, helpful.
Again the voltage measurements around the motor
current loop will tell you.
Pick a ground point and measure from there to every
step around the circuit : battery +, controller +,
controller output, motor +, motor -, battery - ....
Some of this may appear redundant but it's better
to measure something twice, than leave an unknown.
Draw out the power circuit and annotate the voltages.
Now you will see where any drops are - or if none,
other than the motor, that's useful information too.