The devices are linear and symmentric, but I tried to flip the transistor so that the source is placed towards Vdd. There was no difference, as expected.
However I did a transient simulation, and I stand corrected when I said that the PMOS transistor (M79) reduces delay. Actually it increases the delay a bit, but contributed to a signal level increase compared to the situation when the transistor isnt present.
I attached the a graph that shows this. Orange is color is with M79, and purple is without M79. The lower graph is the output of an inverter. As you can see the output of the inverter, when M79 is included is much nicer then the case when M79 is excluded.
Based on simulation results, the case when M79 is included gives much better results when running Monte Carlo simulations at low temperatures (<40 degrees celcius)
I have been searching if this effect might be related to cascode current mirrors, but I cannot find a link that makes sence..