In FP, you will have an immutable variable for state instead of a mutable variable wrapped in a lock. You will be passing a whole new immutable state variable to a pure side effect free function every time, and won't have to worry about getting a lock and releasing it explicitly (these would be side effects). As long as the state is immutable and synced across threads, actors in any thread can just plot their next actions using that state.

In imperative programming, like I said, you'll have to explicitly get and release locks to make sure two actors don't occupy the same spot.

So, if you use atomic immutable variables with pure functions, and the logic in your actors can be conflict free, you can have horizontal scalability across as many cores as you want pretty easily. If your actors cannot be conflict free, you will need to wrap a lock of your choice in a monad and use that, but you will still have gained better debugging, testing and maintainability by using FP.

Now if only every zero-cost OO abstraction had a straight forward FP alternative that was also zero-cost, we'd all be doing FP as of yesterday.