Valid hole fits process is too slow to be enabled by default

I am hearing anecdotal evidence that people are suggesting to turn on -fno-valid-hole-fits as typed holes causes compilation time to double (from 2s to 4s).

@trac-isovector also reports

i have a 140 line file with 17 modules imported: doc <- fromJust <$> #getDomDocument page transformed to doc <- _ <$> #getDomDocument page takes > 1 minute in ghci with the default settings

People just use () rather than _ as _ takes too long. I think we either have to make it more efficient or disable the feature by default.