I'm not an expert on this, and I'm not trying to sound I know everything, but I'm an Indian and have spent 20 years of my life speaking Hindi, which is one of the widely known and spoken language in India, especially in North India. I think this is related to how the language is structured and the way consonants and vowels are used in the "Lipi" (I wasn't able to find an English word for it, but you think of it as the set of symbols with which the language is written.) of Indian languages. The Lipi for Hindi, Sanskrit, Marathi, Bhojpuri, Maithli and many other languages is Devnagari. And It has a somewhat complex structure to it, more complex than English. Like English has 5 vowels and are used directly in the middle of consonants. But in Devnagari, you can see there are traditionally 13 vowels and every vowel can be used independently or dependently in a word, which means you can have a vowel appended or pretended to each consonant, and that will produce a different sound. A kid in India in his early age is taught to identify each of that sound and he uses all that early knowledge and learning, all his life when he talks. This allows him to create and follow different sound patterns and makes his speech continuous and flow-full, which I think you're referring to as being fast. I find other languages like Mandarin has a similar structure, and makes me learn about them even more.
There are a million things I could write here, but thats futile I guess.