nospec for incoherent instances regresses performance
The change in !9411 (merged) to mark incoherent instances as nospec causes less specialisation in optics, inhibiting optimizations and breaking the performance testsuite. See https://github.com/well-typed/optics/issues/488 and #22448 (comment 474141).
I remain of the view that this change should be guarded by a command-line flag and disabled by default. Users who need the additional guarantees requested by #22448 (closed) can then opt in to less specialisation, without imposing a cost on others.