First of all you can set cooking options on a physics mesh but it is a bit vague on the real effect at runtime in terms of performance vs generation speed. I assume “Everything” is the default for runtime created colliders. What " [UseFastMidphase] Determines whether to use the fast midphase structure that doesn’t require R-trees." means to anyone not well versed on physics engines, well who knows! I guess don’t touch anything unless performance blips occur?!
Second I accidently assigned an empty mesh to a new mesh collider and then filled out that mesh afterwards in code This creates an odd situation where the collider is not always updated straight away, it may need to go out of camera view and back in again before it works!! This seems an edge case bug and I will build the mesh first… but any known issue there?