Apache virtual host directives Vs mod rewrites

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I've been trawling the net trying to find an answer to the question:
which gives better performance (both server loading and server response
time), subdomains created via ensim or via mod rewrites?

I've currently gone the Apache virtual host through interacting with
Ensim pro panel, but with my ever growing base of subdomains (each of
my users gets a subdomain, currently at c.1800) I think it's adversely
affecting server performance to a point that soon, irrespective of what
machine I upgrade to I will hit the limit. Then again, there are some
other sites that I'm pretty certain go the mod rewrite route and they
seem to be pretty slow as well.

Anyone with any pointers or thoughts on this would be greatly
appreciated as technically I'm definitely not as good as I think I am.



P.S I know, was silly of me to make the subdomain thing a USP but it
seemed such a good idea at the time

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