A function/part of code is performed say a thousand times, the runtime collects statistics that object ‘a’ was always an integer, so it might be worthwhile to compile this code block to native code with a guard on whether ‘a’ really is an integer (that’s very cheap). The speedup comes from not doing interpretation, but taking the common case and making it natively fast and in the slow branch the complex case of “+ operator has been redefined” for example can be handled simply by the interpreter. Python is not more dynamic than Javascript (hell, python is strongly typed even), which hovers around the impressive 2x native performance mark.

Also, if you are interested, “shapes” are the primitives of both Javascript and python jit compilers instead of regular types.