Are some languages really spoken more rapidly, and some more slowly, than others ?

No. All languages are spoken at pretty much the same number of syllables per second. The feeling that speakers of other languages speak too quickly stems from the fact that listening to a foreign language in which you are not entirely fluent requires you to process the incoming language more slowly. Because you have gaps in your knowledge of grammar and vocabulary and sometimes of pronunciation, it seems that the speakers of the foreign language are always a few words ahead of you. I have been teaching English to speakers of other languages for 30 years and have never met a student of any nationality who didn't complain that English speakers spoke too fast.