Why Vds increase slowly even if it decrease fast?
The output capacitance of the MOSFET has a time constant with the 1K resistor that is around 1usec. You might think it should be more like 360pF * 1K = 0.36usec from the value of \$C_{OSS}\$ on the datasheet, but that low capacitance is measured at a 25V \$V_{DS}\$. 



Referring to Figure 5, we have: 



As to what can be done about it-- it's a bit silly to be using a ~50A MOSFET to switch 12mA. 

Incidentally, the reason it decreases fast is because the capacitance may be the same, but it's being discharged by the MOSFET drain, which can sink perhaps 100A with 12V on the gate. That means the switching speed is dominated by how fast you can drive the gate high through a 40 ohm resistor. 

Use a smaller MOSFET and not only will the gate be easier to drive, but the output will switch snappily. Something like a 2N7000 is more than adequate for 12mA and it has perhaps 1/30 the output capacitance.