What is the relationship between speed and torque in a separately excited DC motor?
No load speed is linear with Voltage 
Torque is somewhat linear with current above excitation currents.
Since stall current is 8~10x rated load current, speed will reduce as load - increases to rated current by more than 10% beyond this exceeds the rated current with conduction losses dominating thermal rise.
This assumes driver and power supply ESR is much lower than DCR of motor coils which is not often the case so speed drops much more than 10%.
since max power transfer occurs when load matches available electro-mechanical power, this max power occurs at half speed


but this may exceed the thermal rating of the motor.

torque declines with rising speed due to back EMF.