Multithreading requests in Rails

I'm pretty new to Ruby On Rails webdevelopment, and I've got the following question:

In my Javascript I launch multiple calls to my controller at once with the use of AJAX, however I'm under the impression these requests get handled one by one, which results in a very slow experience (as some of the requests are quite intense and can take a while to process). I'd expect the server to spawn a separate thread for each request. As far as I'm aware I'm using WEBrick as the server on which my application is running. Online I found some posts indicating that WEBrick is by definition single threaded, so I'm out of luck, however some other posts claim it supports multithreading, but it is prohibited by a mutex in Rails. Most posts seem to refer rails 4.1-4.2, I'm currently running 5.0.1.