[Serum]

perhaps it's not clear to you what this rise delay means;

The speed of a FET/Transistor isn't endless fast. Nor is it's steering signal. (not to mention the polution of the steering signal), this steering signal is obviously is needed to put a fett into full conduction. It simple takes time before the near 0 ohm (full conduction) status is reached. In this time, the FET isn't on it's ideal internal resistance, but the load (motor) stays the same. the fet is at highest efficiency at full conduction. During this 'travel' to full conduction it produces heat (since the internal reistance is getting smaller and smaller reaching full conduction). The speed in which a signal is able to rise, is explained in volts per micro seccond (slew-rate) This rising is done with a steering signal, this steering signal can not be perfect. once the rice took place on a short amount of time, there always is something as a sinus on top of the block-wave, (it's quite impossible to make perfect block-wave) you need the feedback from the signal to correct the errors in the blockwave. (this is the steering signal) not to mention the far from ideal load of a motor (induction) since a controller works on PWM, it's all about different lengths of 'blockwaves'

Now, decide for yourself; if a transistor was digital, would there be a lost? (digital is 0 and 1, and NOTHING in between, since a FET basicly is analog, you have got losses in switching it ON/OFF (1/0)

Hope this explains.