Speed

The situation with computer languages is somewhat like that with human languages:

• Translator:   takes a complete document in one language and creates a complete document in another language, which can then be used at any time.
• Interpreter:   acts as an intermediate between a speaker of one language and a speaker of another language to make it look like both speakers are talking directly to each other.

Using a human interpreter as an intermediate is slower than conversing directly in a particular language. The same is true with computer language interpreters. The interpreter has to do quite a bit of work to deal with the language it is interpreting. The extra work makes it look like the virtual processor is much slower than the real one.