[exim] SMTP processing hangs during bayes sync

I've the following problem with exim and exiscan:

Exim calls spamd in the DATA ACL (for messages from
external hosts).

spamassassin/spamd is configured with

bayes_auto_learn 1
bayes_journal_max_size 0
bayes_auto_expire 0
bayes_learn_to_journal 1

>From time to time (cronjob) the database and the journal are
synchronized (sa-learn --sync). During the synchronization
process SMTP requests are hanging for 20 to 50 seconds (even
those from local hosts, which will not be scanned by spamd).
Number of exim processes are increasing.

Looks like something is locked within exim before spamd
is called, which locks out processing for messages not
undergoing spamd processing.

Any idea how to avoid the delay (dont't want a transport
because we're doing greylisting based upon the spam level)?

kr, Bodo