[exim] Slow email sending and spool file not found

I'm trying to solve a problem with slow email sending with the
following exim version

exim 4.86
Support for: crypteq IPv6 Perl OpenSSL move_frozen_messages
Content_Scanning DKIM Old_Demime DNSSEC PRDR OCSP Experimental_SPF
Experimental_SRS
Lookups (built-in): lsearch wildlsearch nwildlsearch iplsearch cdb dbm
dbmjz dbmnz dnsdb
Authenticators: cram_md5 dovecot plaintext spa
Routers: accept dnslookup ipliteral manualroute queryprogram redirect
Transports: appendfile/maildir/mailstore/mbx autoreply lmtp pipe smtp

What happens is that email sending gets stuck at "Connecting" (when
using Thunderbird) for a long time before delivery is made. Comparing
exim -bd -d output on the problem server and a "normal" server, the
problem seems to be this:

The problematic exim install is doing dns lookup for every single
recipient and does not continue until it finishes. At 5~15 sec per
recipient, this becomes an enormous lag.

On the normal install, exim starts looking up recipients but does not
hang on to the connection and Thunderbird reports mail sent even while
Exim is still doing dns lookups.

I've tried turning hosts_lookup off as well as set
rfc1413_query_timeout = 0s but neither has any effect.

I can't figure out what else could be causing the difference. Could an
ACL be causing this? Or is there a setting to force exim not to hold
up the sending while it is looking up recipients?