Speed difference in Gnus

The speed difference between XEmacs and GNU Emacs is amazing, in that GNU Emacs is significantly faster, at least for "heavy" usage such as Gnus. Entering a large summary buffer takes time proportional to the *square* of the number of messages in the folder. (This is not a wild guess, a friend has actually measured.) Time to enter a folder of more than 3000 articles quickly degrades to minutes. For the longest time I believed this to be a result of XEmacs's implementation of text properties on top of extents. I planned to convert Gnus to use extents natively, in the hope that it would speed up summary buffer generation. But according to profiling, most of the time is spent in `insert', at least once you discount excessive GC by increasing gc-cons-theshold. I'm not sure what XEmacs is doing wrong here. I suspect Mule is to blame, but I can't prove it. Using FSF Emacs gave me an order-of-magnitude speedup when entering large newsgroups and large mailing list folders. It's uncanny.