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.