How are Java threads heavy compared to Scala / Akka actors?

Scala actors (including the Akka variety) use Java threads. There's no magic: more than a few thousand threads running simultaneously is a problem for most desktop machines.

The Actor model allows for awake-on-demand actors which do not occupy a thread unless they have work to do. Some problems can be modeled effectively as lots of sleeping agents waiting to get some work, who will do it relatively quickly and then go back to sleep. In that case, actors are a very efficient way to use Java threading to get your work done, especially if you have a library like Akka where performance has been a high priority.

The Akka docs explain the basics pretty well.

All reasonably scalable web servers have to solve this sort of problem one way or another; you probably ought not be basing your decision for web server primarily on whether actors are used under the hood, and regardless of what you use you can always add actors yourself.

An Akka actor is not equivalent to a thread. It is more like a Callable that is executed on a threadpool.

When a message is dispatched to an actor, that actor is placed on a threadpool to process the message. When it is done, the pooled thread can be used to execute other actors.