"My bet is that nobody thought about it."

Well, you lost that bet. You're not the first person in history to think about what language is the slowest and it's a question that can be found in many language forums. A study showed that Mandarin is a relatively slow language (syllables per minute) whereas Spanish and Japanese are fast. But the information density is high in Mandarin, so even though it's spoken significantly slower than other languages, it offers the same amount of information in the same amount of time:

<a href="https://www.scientificamerican.com/article/fast-talkers/" target="_blank">https://www.scientificamerican.com/article/fast-talkers/</a>;

<a href="http://www.thelinguafile.com/2015/04/speech-tempo-what-is-worlds-fastest.html#.XVAO8ugzY2w" target="_blank">http://www.thelinguafile.com/2015/04/speech-tempo-what-is-worlds-fastest.html#.XVAO8ugzY2w</a>;

Also, one can't just say that a language is fast or slow per se. It also depends on regional accents. Swiss people speak French significantly slower than French people, even though both are native speakers.