Using exim4-daemon-heavy to use the exiscan patch that allows to use spamd filtering in acl's fails
with large emails.  I don't know the exact cutoff but with a 4.5M email that a command line invocation
of spamc can process in < 1 second, spamd takes about 10 minutes at max cpu for one core.

This is clearly a flaw in the way exim/exiscan is handling the email rather than a limitation
of spamassassin/spamd.

The email in question is just a firewall log summary from repeated blocks of a misconfigured program
and is a pure text file of kernel log entries.