Throwing httperf or ab at a single mongrel instance will give you an idea what the theoretical average request/response rate should be per server. And it would probably be just fine for a single user, right? But a single mongrel may not be serving up enough requests to give you the concurrency you need. In that case, you should load balance to a mongrel cluster using pound or pen or mod_proxy_balancer. See if firing up 3-5 mongrels and load balancing that way helps out.
If you are still in "setup" mode, then you may be doing some premature optimization (I hate it when people say this to me :). You might want to get a good grasp on your performance bottlenecks, and then examine a combination of caching, serving static pages directly from Apache, alternate "light stacks" like merb to speed things up.