One of the benefits of Scala's willingness to break bytecode compatibility all the time is that whenever Java/JVM implements a Scala concept, Scala switches to the Java method under the hood. Whenever the JVM gets Java-specific optimisations, Scala will use those. That's honestly pretty cool.

They have in practice been slower because they try to be higher level, more ergonomic languages than Java. There aren't many performance-first JVM languages yet I think (Fortress maybe? They cited JVM problems when they wrapped up though).