[GHC] #14052: Significant GHCi speed regression with :module and `let` in GHC 8.2.1

[GHC]

#14052: Significant GHCi speed regression with :module and `let` in GHC 8.2.1
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.4.1
       Component:  GHCi              |              Version:  8.2.1-rc2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 To make things worse, this script takes even longer between GHC 8.2.1 and
 HEAD. As noted above, on 8.2.1 it takes about 35 seconds, but on GHC HEAD
 (as of commit d08b9ccdf2812e8f8fa34d0c89275deee574524c), it takes about 2
 minutes 32 seconds. I still need to investigate to which commit(s) were
 responsible for this further regression.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14052#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler