Some languages seem to rattle away at high speeds, and others sound slow. This also happens with dialects, with for example West Americans speaking slower, maybe even with a drawl, and urban accents from for example London or New York are considered fast. However, this is often untrue. When listening to an unfamiliar language, you can barely even judge it fairly. Can you even scientifically prove that some languages are faster than others? 

There are three options:

1. Some languages are really spoken faster, and others slower.

2. We get the impression that they are spoken faster because of an illusion

3. In some societies it is acceptable to speak faster, and in others it is not. 

To get straight to the case:

1. There is no real difference, but some are slightly different. It varies per person per occasion though.

2. Again, no real difference. There is no real illusion going on when you listen to a foreign language. Interestingly enough, Dutch has the highest speaking rate, even though it is not thought of as a very fast language.

3. This varies per language. In Zulu, slow talkers are considered respectful and sincere, but in Italian it is the best to switch from slow to fast, to increase communicative value.

There is no real answer yet, but Three definitely seems the most plausible.