H-Bridge diode makes the motor go faster
My best guess is that the motor speed controller is using the back-EMF of the motor as feedback to control the current it feeds to the motor. 

By preventing it from seeing the back-EMF (blocking it with a diode), it thinks the motor is not turning, so it cranks the current up (and up). The delay and the ramping up time and rate are related to the control algorithm and the tuning of the control loop. 

You may be able to get it to work by paralleling the diode with a resistor of just high enough value that the motor won't turn in the wrong direction, but hopefully low enough that the controller can still read the back-EMF.