Hi,

So here is the crux of this issue.  When using
'gnus-summary-refer-thread' in a nnml group, Emacs ends up calling
'gnus-get-newsgroup-headers-xover' (via 'gnus-fetch-headers').  AFAIU in
this function when 'gnus-read-all-available-headers' is t, Emacs will
parse *all* of the " *nntp*" buffer content.  In my case, this buffer is
quite big (about 50k lines and 23MiB) hence the slowness.

BTW, I also have examples where 'gnus-summary-refer-thread' gives me
some false positives (eg., not the same thread but part of the subject
matching)

I don't know how to proceed except from unsetting
'gnus-read-all-available-headers' for nnml backends (see attached
patch).
-- 
Manuel Giraud