On 2017-12-07 03:33 PM, Brandon Allbery wrote:

Certainly the behavior here sounds wrong and should be reported.

https://ghc.haskell.org/trac/ghc/ticket/14564 - there’s already been a response (from Simon P-J) and it is being treated as a bug.

I get the slowdown with 8.0.2, 8.2.1 and 8.2.2 but 7.10.3 is OK.

Since 8.2.2 is very recent (20 Nov) I didn’t go to the trouble of building ghc from source.