MOSFET delay time when transitioning between triode and saturation regions. Charging and discharging mosfet
The different transition times are a result of your particular test setup.

The transition time depends on the parasitic capacitance at the output and the current to charge or discharge it. In your case, for a falling edge discharging is done by the transistor itself and a large current is possible. 

In case of rising edge the transistor turns off and the current has to be supplied by the 10uA current source. Because of the resistor at the output this results in an exponential characteristic.