You can write Java-like code in Scala that compiles down to nearly identical bytecode to the Java version. If you're not using any primitives it will be 100% as fast.
If you write idiomatic Scala code, or use many primitives in Java code, the performance of Scala becomes relatively worse. Every first-class function you use turns into the equivalent of a new class and object at runtime, and the immutable data structures are slower than their Java equivalents.
I'm not sure how the performance delta differs between the JVM and Android's Dalvik VM. It may be that Dalvik's lack of JIT or less efficient garbage collector makes the Scala performance penalty bigger on Android. I would be interested in seeing some benchmarks.
If you write idiomatic Scala code, or use many primitives in Java code, the performance of Scala becomes relatively worse. Every first-class function you use turns into the equivalent of a new class and object at runtime, and the immutable data structures are slower than their Java equivalents.
I'm not sure how the performance delta differs between the JVM and Android's Dalvik VM. It may be that Dalvik's lack of JIT or less efficient garbage collector makes the Scala performance penalty bigger on Android. I would be interested in seeing some benchmarks.