[Dovecot] Slow caching

[John Rowe]

I have just installed dovecot on a RedHat Enterprise 4 clone with exim
MTA, procmail LDA and mbox format. I'm getting complaints of slow
caching from a Mac user:

        I think there's still something very sick with [host]'s IMAP 
        server: it just took a full five minutes to cache an attachment
        (350kb) 
        in an email from [person]. In that time, I couldn't do anything
        else with 
        mail; it was essentially locked.
        
The user's inbox is in /var/spool/mail (where there are no idexes),
there areindex files within his home directory:

./.imap/INBOX:
total 1160
-rw-------  1 xyz users  60656 May 30 14:22 dovecot.index
-rw-------  1 xyz users 891904 May 30 14:22 dovecot.index.cache
-rw-------  1 xyz users  78428 May 30 14:22 dovecot.index.log
-rw-------  1 xyz users 131920 May 26 11:29 dovecot.index.log.2

Everything is on local disk, there are no NFS mounts at all. Any
thoughts?

Thanks

John