[Dovecot] Slow caching

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