MOSFET fall time too slow
Two things.

First, there is no reason for a 1 k&Omega; resistor between a 5V digital output and the FET gate, especially such a large resistor.  There are certain cases when switching large loads where you do want to provide some finite impedance driving the gate, but that is not necessary here and the minimum impedance of the digital output is likely more than that anyway.

Second, what you are really seeing is not the FET turning off slowly (althouth the large gate resistor does cause some of that), but rather that there is nothing but the scope probe driving the source low after the FET has turned off.  You are seeing the voltage on the inevitable capacitance on that line, which includes the scope probe capacitance, being discharged only by the high impedance of the scope probe.  For a typical 1x probe, that is 1 M&Omega;.  If it's a 10x probe, then there is probably only 10 M&Omega; discharging the aggregate capacitance.  Note that even 1 M&Omega; and 100 pF is a time constant of 100 &micro;s.