In #10291 we added a hack to serialize the execution of transpile() with multiple circuits if a custom InstructionDurations or dt was specified as arguments to transpile(). This was to workaround the behavior that the InstructionDurations object constructed by transpile() was a function of the input circuit to factor in the durations of custom calibrations in the circuit to that custom InstructionDurations object. A better long term fix is to revise the passes using InstructionDurations to do the custom circuit calibrations handling inline as needed instead of doing it in transpile().

I do wonder if an implementation of #9256 would alleviate some of those concerns. Because then the passes will only get a target and we're not using InstructionDurations anywhere (except for the plugin interface). But I'll convert this comment into an issue for 0.26 and we can track it there.

Originally posted by @mtreinish in #10291 (comment)