Semantics aside, there is really no difference between performance in Scala and Java, equivalent code in both compiles to near identical byte code, save for a few niche optimisations (eg tail call recursion) that can make Scala faster. I discussed this here:

https://jazzy.id.au/default/2012/10/16/benchmarking_scala_ag...

Since it's all just bytecode, then there's no difference in what you can do when it comes to scalability. However, it's the semantics, that you think are just personal preference, of Scala that make writing scalable code easier, it biases you towards scalable code with things like default immutability and using asynchronous io and things like that. Anything you do in Scala you can do in Java, but the question is, when the syntax overhead of for example monadic programming is over 5 times greater than the equivalent scala code (and many times less readable and maintainable), would you do that? Most developers don't. I wrote another blog post about that here:

http://jazzy.id.au/default/2012/11/02/scaling_scala_vs_java....