.class files are a fairly simple mechanical translation from java into bytecodes making them easy to decompile. Many optimizations that a compiler would do for static code are not done as the JIT can decide at runtime.

The Java->OpenCL framework https://code.google.com/p/aparapi/ decompiles kernels written in Java class files back into algebra and reprojects them into OpenCL.

The peformance of a LuaJIT based JVM could actually be quite good depending on how much work is done in the decompilation stage.