When using domain mapping with Wordpress every single page view is doing
a domain mapping within the database. We have atleast two (2) database
queries for every page request.

As domain mapping probably never ever will change in your site installation 
this information from db could be cached. I use a cache time for 14 days in
my modification, but you could probably set it to a year.

If you use this modification I recommend you to mount /tmp in RAM (tmpfs) as 
there will be a lot of reading from those files ;)

Have a lot of fun!
Jonas Bjork, [email protected], 2011-05-13