If you are thinking on doing web apps, I think Clojure is the best option, as it enables you to write clojure to target both server (either in clojure or clojurescript) and browser (with clojurescript, that compiles to javascript), and share code between those seamlessly (for example, protocols, specs, business logic, etc. that you may need in both sides).
That's the most extended option by far, but I'm sure that you will find other lisps that compile to javascript (not that I know them).
> 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.
The "web apps" problem is the most important one, I think. The short answer for us was the strangeness of building HTML or GUI UI's with Lisp. The solution we used was to make our UI in HTML/JS/Angular and have it talk to a Lisp backend with Node.js.
In this way, we use Lisp for what it's good at and use HTML/JS to make a modern front end.
I am seriously thinking of learnijg common lisp. Thank you for all the suggestions.
Is it possible to build web apps in common lisp or any lisp? Any frameworks available?
Is it possible to build multithreaded apps that use multiple CPU cores for performance? What is used? POSIX threads?