Like always, the slew rate δV/δt = Iout/Cout .
The current Iout of the output buffer is the adjusted current Ipre of the pre-amp stage (through M12, M10 ...) times the current gain by the W/L ratio(s) between the output- and pre-amp-stage Iout/Ipre = (W/L(MON))/(W/L(M10)) , or the ratio of the corresponding p-channel transistors - if different, take the smaller one for the Iout calculation.
Cout, self-evident, is the load capacitance Cload, plus the feedback capacitance (= compensation cap, in your case) times this gain factor (if different for p- and n-channel, take the bigger one, in this case).