[exim] Performance of MySQL vs text files.

I'm looking at developing a system where domains and users for virtual
domains are specified and then transferred over to my mail server. I
can do what I want on the mail server separate from the info on the
configuration server.

What I want to know is whether it would be quicker to store everything
in a MySQL database (or PostgreSQL or anything really, I'm not fussy) or
in a flat text file. What I mean is relaying mail in a file like:

foo.com: 192.168.4.1

or put it in a MySQL database.

When I say quicker, I mean for exim to use, not development.

TIA,

-John