Have a look at these performance comparisons. Groovy++ can be nearly as fast as Java and Scala.
Project lead Alex Tkachman gives some more backgrounds about the project in an interview at dzone:
The good news is that we don’t need fully dynamic behavior always. Great syntactical expressiveness plus powerful type inference can do magic with statically compiled code. Here the idea of statically typed groovy comes in to play. Let us separate code which requires high performance from code which requires fully dynamic features.
We get a very expressive language, which is similar and close to Java from one side and from another side, where different pieces of code can enjoy performance and compile-time type checking (critical for integration with existing Java code) and other parts can be fully dynamic. This is the main goal of Groovy++.