We have a large password file. Can Exim use alternative lookups during delivery to speed things up?

If you are using FreeBSD, this problem should not arise, because it automatically uses an indexed password file. In some other operating systems you can arrange for this to happen too. On Linux, for example, all you need to do is

# cd /var/db
# make

and put db before files in any /etc/nsswitch.conf lines you want to use db for. On systems that do not include support for indexed password files, you can build one yourself, and reference it from the Exim configuration. For example, for routing to local mailboxes you could use this:

localuser:
  driver = accept
  condition = ${lookup{$local_part}cdb{/etc/passwd.cdb}{yes}{no}}
  transport = local_delivery
  user = ${extract{1}{:}{${lookup{$local_part}cdb{/etc/passwd.cdb

}}} This assumes a cdb version of the password file.