"setoid_subst" call takes forever

This line takes forever to execute on Coq 8.11 (don't try Coq 8.12, that doesn't even get there, see here). We are talking on the order of minutes here.

The ofe_subst a bit later seems to be the same.

I don't know when this started (we do not have timing data for this repository), but I doubt it was always like this.

@robbertkrebbers any idea what is happening?