It's a bit of both. Julia's JIT makes code without annotated types fast since it can figure out the types just before runtime and de-virtualize on that basis.

It's a bit of both. Julia's JIT makes code without annotated types fast since it can figure out the types just before runtime and de-virtualize on that basis. The reason julia is able to make such aggressive compile time optimizations is because it only compiles the code that you actually use. In many ways, julia (performance-wise) is similar to heavily templated C++, but the key difference is that in C++ templated code, you have to compile for every combination of possible types, which can easily be thousands of times more methods that are actually needed.