Lifting a function from where clause to top level causes compilation time to triple

I have this program that depends on the library "red-black-record" version 2.0.2.2 on Hackage:

{-# LANGUAGE DataKinds, TypeApplications #-}
module Main where

import Data.RBR (FromList,Delete,Variant,I,injectI,winnowI,match)

import GHC.TypeLits

type Phase01 = FromList '[ 
      '("ctor1",Int),   '("ctor2",Bool),  '("ctor4",Char),  '("ctor3",Char),
      '("ctor6",Char),  '("ctor5",Char),  '("ctor10",Char), '("ctor11",Char),
      '("ctor13",Char), '("ctor14",Char), '("ctor39",Char), '("ctor46",Char),
      '("ctor47",Char), '("ctor44",Char), '("ctor43",Char), '("ctor7",Char),
      '("ctor9",Char),  '("ctor20",Char), '("ctor45",Char), '("ctor21",Char),
      '("ctor48",Char), '("ctor49",Char), '("ctor50",Char), '("ctor41",Char),
      '("ctor33",Char), '("ctor32",Char), '("ctor42",Char), '("ctor22",Char),
      '("ctor23",Char), '("ctor8",Char),  '("ctor40",Char), '("ctor29",Char),
      '("ctor24",Char), '("ctor38",Char), '("ctor25",Char), '("ctor26",Char),
      '("ctor27",Char), '("ctor28",Char), '("ctor36",Char), '("ctor52",Char),
      '("ctor51",Char), '("ctor53",Char), '("ctor12",Char), '("ctor54",Char),
      '("ctor15",Char), '("ctor31",Char), '("ctor30",Char), '("ctor34",Char),
      '("ctor35",Char), '("ctor17",Char), '("ctor16",Char), '("ctor18",Char),
      '("ctor19",Char), '("ctor37",Char)
      ]

type Phase02 = Delete "ctor1" Int Phase01

main :: IO ()
main = print (match @"ctor17" (fromPhase1ToPhase2 (injectI @"ctor1" 2)))
    where
    fromPhase1ToPhase2 :: Variant I Phase01 -> Variant I Phase02
    fromPhase1ToPhase2 v = case winnowI @"ctor1" @Int v of 
        Right z -> injectI @"ctor2" False
        Left l -> l

"red-black-record" provides extensible variants; the code is basically removing a branch from a variant with 50-plus branches, and then trying to match another branch. It is type family-heavy code.

The code as it is takes **~9 seconds** to compile on my machine. But when I move the fromPhase1ToPhase2 function to the top level (including the signature) compilation time balloons to **~ 29 seconds**. Is there a reason it should be so?

As another datapoint, moving the function to the top level but omitting the complex type-level map parameters (Phase01, Phase02) using partial type signatures (also requires a new type application) compiles in **~9 seconds** again.

{-# LANGUAGE PartialTypeSignatures #-}
{-# OPTIONS_GHC -Wno-partial-type-signatures #-}

-- ...

type Phase02 = Delete "ctor1" Int Phase01

fromPhase1ToPhase2 :: Variant I _ -> Variant I _
fromPhase1ToPhase2 v = case winnowI @"ctor1" @Int @Phase01 v of 
    Right z -> injectI @"ctor2" False
    Left l -> l

main :: IO ()
main = print (match @"ctor17" (fromPhase1ToPhase2 (injectI @"ctor1" 2)))

Trac metadata

Trac field	Value
Version	8.4.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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