Certainly your web server isn't likely to be the performance bottleneck in the majority of real-world applications. Even without a db call, just generating the response Clojure-side will kill your numbers (and Clojure is pretty darn fast).

Still, the ability to cheaply handle large numbers of concurrent connections is great for things like WebSockets and long polling where you aren't crunching data for every connection all the time. This is an async server, so it's well suited to such tasks.