Both Python and Java compile the source files to bytecode. The difference is in how they to run this bytecode. In both languages, the bytecode is basically a binary representation of the textual source code, not an assembly program that can run on a CPU. You have a different program accepts the bytecode and runs it. How does it run it? Python has an interpreter, i.e a program that keeps a "world model" of a Python program (which modules are imported, which variables exist, which objects exist...), and runs the program by loading bytecodes one by one and executing each one separately. This means that a statement such as y = x + 1 is executed as a sequence of operations like "load constant 1", "load x" "add the two values" "store the result in y". Each of these operations is implemented by a function call that does something in C and often reads and updates dictionary structures. This is slow, and it's slower the smaller the operations are. That's why numerical code in Python is slow - numerical operations in Python convert single instructions into multiple function calls, so in this type of code Python can be even 100x slower than other languages. Java compiles the bytecode to machine code. You don't see it because it happens at runtime (referred to as JIT), but it does happen. Since Java also knows that x in y = x + 1 is an integer, it can execute the line using a single CPU instruction. There's actually an implementation of Python that also does JIT compilation. It's called PyPy and it's five times faster than CPython on average, depending what exactly you do with it. It will run all pure Python code, I think, but it still has problems with some libraries. Answer from unruly_mattress on reddit.com