tonfa:
> It the 2x slowness acceptable ? It's the cost of having a more precise 
> rename detection algorithm.

Err, I guess it's OK.  But remember, I'm running diff on a single filename.  
I don't see why rename detection enters the picture.  If I was diff'ing two 
whole changesets, sure, that would make sense... but two revisions of a 
single file?

pmezard:
> Do you have a large working directory? If so, it could be related to
> issue1955 too.

Not especially: I found this in Mercurial's own source tree, with ~1100 
files and ~10 MB in the working dir.  Not big.