slow "standard mailing list" aggregation

My inbox has about 28,000 messages in it, and when I enter that folder, kmail 
1.12.2 takes about 25 seconds to process, thread, and group all those 
messages.  For this same folder (it's on an IMAP server), kmail 1.9.10 takes 
about 3 seconds.

kmail 1.12.2 is only slow if I use "standard mailing list" aggregation, which 
seems to look the same as the way I have kmail 1.9.10 configured.

I really like the standard mailing list aggregation.  Is there some caching or 
tweaking I can do to make the performance acceptable?

Bjorn