The corresponding motor speed will be lower with a lower cadence (as crank speed and motor speed will be directly related).
At lower motor speeds, when under load, the motor is pulling current and turning as hard as it can trying to accelerate, but because of the low cadence, it's not spinning fast enough to work effectively. Ultimately, you're putting more strain on everything within the motor by loading it and at the same time you're converting more of your battery power into heat rather than forward motion.
It's not the same motor, so the figures won't be the same, but on the Kenevo I recently noticed that riding with a damaged achilles tendon I subconsciously dropped my cadence to 60-70 which resulted in a 30% range drop. Back above 80 and everything is normal again.