[exim] exim taking long time to respond

I am trying to validate my new email server, and I'm finding that it's
taking about 6 seconds to respond and 8 second to process an email.

The server is lightly loaded so there's something that is causing this
delay. AFAIK I have no issues with DNS. I am using dspam+spamassassin,
so that could be the cause of the delay, but this is on a quad-core
server, ssd drives, and load of typically around 1% so it should be much
faster. At a guess there's some lookup that's taking that long.

Any way to tell where the delay is coming from?

Here's a typical transaction - note the time stamps. 10 seconds from
start to finish.

15-10-03 05:35:07 1ZiM1r-0002rf-DI DKIM: d=lightinthebox.chtah.com
s=20111013 c=relaxed/relaxed a=rsa-sha256 t=1443875689 x=1459686889
[verification succeeded]
2015-10-03 05:35:13 1ZiM1r-0002rf-DI <=
bo-b8f3zs6bfs4q1kaupz7fabytuz0ep2@???
H=mta925.chtah.net [8.7.42.21] P=esmtp S=96956
id=b8f3zs6bfs4q1kaupz7fabytuz0ep2.1951771.9982@???
2015-10-03 05:35:17 1ZiM1r-0002rf-DI => yan <yan@???>
R=spamcheck_director T=spamcheck
2015-10-03 05:35:17 1ZiM1r-0002rf-DI Completed