Why is SLEW RATE in nMOS amplifiers worst when the output is going from LOW to HIGH?
Turn-off slew rate depends on load current only as each switch opens.  Turn-On slew rate is fastest due to T=RC when the switch is on at R=RdsOn.

The same is true for collector slew rates in a BJT transitor.  Turn off time depends on the pullup-R * C =T where C can be stray capacitance to ground.

The 1st principal is $$dV/dt=Ic/C=V/RC$$ For any load capacitance including FET Coss ( which also increases with power rating and RdsOn value).  

So an open drain turns off the slowest into a high R load. While reducing Ron may or may not increase slew rate so the product of RdsOn and Coss is often a Figure of Merit (FOM) for power FETs.

Since Nch switches ON low it, therefore, relies on the Pch to turn ON after some dead-time to pull up fast.