traceTc is inefficient

The current definition of traceTc:

traceTc :: forall a. String -> Doc a -> Tc ()
traceTc fname message = do
  path <- getTraceOutputPath
  case path of
    Nothing -> pure ()
    Just x -> liftIO $
      do actPath <- makeAbsolute x
         exists <- doesPathExist actPath
         let prettyMessage
               = renderStrict
               . layoutPretty defaultLayoutOptions
               . (<+> line)
               $ vcat [pretty fname <+> lbrace
                      ,indent 1 message
                      ,rbrace]
         if exists
           then T.appendFile actPath prettyMessage
           else T.writeFile actPath prettyMessage

Use a proper logging mechanism instead.