[ballpark]

Yes, Clojure

> Is it possible to build web apps in common lisp or any lisp? Any frameworks available?

Clojure would be good because it has lots of users. Many web apps in production. It might be good to start out with luminus for Clojure, but maybe even before that, try to get a simple Ring and Compojure thing going where you can send a request and get feedback.

> Is it possible to build multithreaded apps that use multiple CPU cores for performance? What is used? POSIX threads?

Clojure is awesome for multi-threading. That's one reason Clojure is functional/immutable by default.

Also, you get access to many libraries in the language hosts (Java, Javascript). That could be a big reason for its success.

Good luck in your pursuit!