Transient response to rectangular impulse of a cascode common source amplifier
This is a very high-impedance circuit, and this means that parasitic effects &mdash; particularly parasitic capacitances &mdash; are going to be important.

Specifically, the upper transistor has a significant capacitance associated with its drain terminal. When the transistor turns on, it can discharge this capacitance very quickly, because the current flow to ground is essentially unlimited. However, when the transistor cuts off, you only have a fixed current source charging this capacitance.

Qualitatively, you would expect to see a linear ramp of the capacitor voltage, but in your situation, the load resistance "steals" more and more of the available current as the node voltage rises, so you end up with an exponential curve whose shape varies with the load resistance. A smaller resistance gives a shorter R-C time constant.