[Dovecot] body search very slow since upgrade from 1.0.15 to 1.2.10

[Stan Hoeppner]

Did you mislead me Timo?  You said search in 1.1+ is faster than 1.0.  I'm
seeing approximately 20x *slower* search times in 1.2.10.

Via Thunderbird, a full body search of my 11,000+ message IMAP folder hosted
by 1.0.15 used to take less than 10 seconds.  Since upgrading to 1.2.10 the
search is taking over 3 minutes, the imap process servicing the client
pegging one CPU at 100% for the duration of the search.  Here's the mbox file:

-rw-------  1 stan stan  50M Feb 23 07:41 1-Spam-l

Doing the same search via grep results in a time of:

real    0m0.361s
user    0m0.204s
sys     0m0.140s

grep takes less than half a second, yet dovecot 1.2.10 takes 3 minutes for
the same search?  Actually grep is searching the entire file including
headers, so it's doing more work than the imap process.  Something is
*SERIOUSLY* wrong here.

Did I somehow nerfed something in dovecot.conf that's causing this?  I
deleted all the index files as per the upgrade instructions.  However,
checking the directories shows they're apparently rebuilt:

[08:09:15][root at greer]/home/stan/mail/.imap/1-Spam-l$ la
total 28M
drwx------  2 stan stan 4.0K Feb 23 07:41 .
drwx------ 35 stan stan 4.0K Feb 15 11:14 ..
-rw-------  1 stan stan 358K Feb 23 07:18 dovecot.index
-rw-------  1 stan stan  27M Feb 23 07:41 dovecot.index.cache
-rw-------  1 stan stan  616 Feb 23 07:41 dovecot.index.log

What do I need to check/change/do to get these search times back down to
reasonable/respectable levels, i.e. around 10s?

Thanks.

-- 
Stan