How faster is mod_rewrite, compared to Zend Router? Should one give up using Zend Router and do the same things with rewrite rules in apache's virtual host definitions using mod_rewrite?
Is the difference very significant?
I have heard many people saying mod_rewrite has caused some high traffic websites crash, I was wondering if this should be of concern when developing with ZF
It sounds to me like you're heading towards premature optimisation. Whilst routing only with mod_rewrite might be slightly faster than mod_rewrite + ZF routing, is a 1-2ms speed increase (probably less) really worth the sacrifice in maintainability and flexibility? You should only consider such a course of action if you have an application where you know (through benchmarking) that routing is your bottleneck, and you've already exhausted all of the standard solutions to such a problem.
As for mod_rewrite itself, I've seen it crash sites but only when the rules were badly written (causing it to rewrite the same request multiple times). This isn't really something you need to be concerned about if you're just using ZF's default rule, which simply rewrites all requests to one PHP file.