For the tool, I'm using a client/server model, and on the client side latency is a huge concern, so I'm using Java. Java is basically assembler for the JVM, and has extremely predictable performance characteristics. Where performance is a non-issue, or I only care about throughput, I'm using Clojure, because it's much more expressive. I think the tradeoffs of the two languages are very complementary, but also much less extreme than, say, Ruby and C.
I find the inertia of the core implementation of the language annoying, even if it's not a huge problem for most applications. Clojure's immutable data structures were world-class when the language was first released ten years ago, but there have been a number of papers which detail improved approaches since then. I've implemented most or all of them [2], but the chance of getting these back into Clojure is effectively nil. That doesn't affect me (I can just use my own data structures), but it does give me some concern as to where the language will be after another ten years.