The language and its implementation will limit the efficiency of the end-result (the running code).
The current Google Go compiler implementation produces slow code, but there's a potential to make it as fast as C.
The JavaScript implementation can be made better, but it will never be close to C.
Scala running on JVM will produce similar efficiency to Java. On multicore the Scala compiler will likely produce a faster code by default, but that advantage can be matched in Java with extra work.
Scala running on .NET will likely perform similar to C#.
Now it's up to the developer to create efficient source code for the given language. It is possible to write slower code in C than in JavaScript, but the focus should be on "normally".
Normally Scala gives you the same end-result with less work than Java.
There are exceptions.
The current Google Go compiler implementation produces slow code, but there's a potential to make it as fast as C. The JavaScript implementation can be made better, but it will never be close to C.
Scala running on JVM will produce similar efficiency to Java. On multicore the Scala compiler will likely produce a faster code by default, but that advantage can be matched in Java with extra work.
Scala running on .NET will likely perform similar to C#.
Now it's up to the developer to create efficient source code for the given language. It is possible to write slower code in C than in JavaScript, but the focus should be on "normally".
Normally Scala gives you the same end-result with less work than Java. There are exceptions.
More: http://www.scala-lang.org/node/25 (Look for "Scala Compiler Performance")
An older measurement: http://fupeg.blogspot.com/2008/06/scala-vs-java-performance....