Well, they're both implemented in LLVM, but Julia uses a JIT and not straight to a native binary. I bet they have very similar performance. The use cases are different though. Julia is a replacement for MATLAB, R, Fortran, & scientific Python with speed and good macro support. Crystal is basically fast/native Ruby, so websites and business apps. You could technically use either for those applications, but the communities will be mostly scientific computing for Julia and business apps for Crystal.