Actually fewer syllables per second is good, means you’re spending less effort speaking. It’s the ratio of information/syllables you want to maximize. Which means German/English/Mandarin/Vietnamese are roughly on par as the most “efficient” languages.
Some languages have fewer vowel sounds while others have an insane number (in Europe that would be Danish).
Thai has a lot, so speakers need to speak more slowly so the listener has time to distinguish words. But it also means that you can have more words per syllable.
It's not about efficiency per se - it's data and error correction
Just to add - Thai has a tonal system and distinguishes rising, low, medium, high and falling tones. This requires a bit more time to say so that there is time for the tone to change (or not change).
In practical speech though, the tones get reduced to something like 2-3.
My opinion on why Thai shows up this way is that pronouns and articles are often omitted, and a lot of meaning comes from very short end of sentence particles, and different vocabulary for different registers.