Speed of RewriteRule?

I have a site that is roughly one half "my" site, and one half a SAS site from a third party.

We've been trying to tackle a single sign on method for many moons.

It dawned on me recently that if I did some URL rewriting to display the third party site, but appear on 开发者_如何学Pythonour domain, the third party cookies would be written as coming from my site, and nullify any cross site scripting issues.

Is there any performance hit that I should be cautious about here? I use rewriting all over the place to hide file extensions and that sort of thing, and the end user does not notice a difference. Should I expect similar performance running an entire site/app through a proxy?

Thanks for any help.