Why are Interpreted Languages Slow?

Question

I was reading about the pros and cons of interpreted languages, and one of the most common cons is the slowness, but why are programs in interpreted languages slow?

Answer 1

A simple question, without any real simple answer. The bottom line is that all computers really "understand" is binary instructions, which is what "fast" languages like C are compiled into.

Then there are virtual machines, which understand different binary instructions (like Java and .NET) but those have to be translated on the fly to machine instructions by a Just-In-Compiler (JIT). That is almost as fast (even faster in some specific cases because the JIT has more information than a static compiler on how the code is being used.)

Then there are interpreted languages, which usually also have their own intermediate binary instructions, but the interpreter functions much like a loop with a large switch statement in it with a case for every instruction, and how to execute it. This level of abstraction over the underlying machine code is slow. There are more instructions involved, long chains of function calls in the interpreter to do even simple things, and it can be argued that the memory and cache aren't used as effectively as a result.

But interpreted languages are often fast enough for the purposes for which they're used. Web applications are invariably bound by IO (usually database access) which is an order of magnitude slower than any interpreter.