It seems pretty good. Twitter has some talks on this where they claim signifiant performance improvements: https://www.youtube.com/watch?v=PtgKmzgIh4c.

I've heard that the Twitter JVM team has a road show where they've talked to some other large Scala users about the performance improvements. Initially, people are highly skeptical of the claims, but after trying Graal on their internal workloads, they generally see similar results.

Here's a paper which has some explanations for why you might expect Graal to improve Scala performance: http://aleksandar-prokopec.com/resources/docs/graal-collecti...

If Graal and Scala interest you, then you should be aware of the Scala Native project:

http://www.scala-native.org/en/v0.3.9-docs/ https://github.com/scala-native/scala-native

It provides manual memory management as needed, making it suitable for even hard real time applications.