parasitic extractions cworst
There is no clear relationship between RC corners and maximum delay. Some nets are resistance-dominated and some are capacitance-dominated and some are a mix. Add to that the fact that increasing delay is bad for setup timing but good for hold timing, and vice-versa for decreasing delay.
So all you can say is that by picking different R and C combinations you will get a different delay picture for your circuit. Not necessarily better or worse, just different.
Foundries try to restrict the number of corners people need to verify, so they pick two corners that are typically best and worst. Just keep in mind it is a simplification of what really happens.