One way of classifying languages is grouping them into stress-timed, syllable-timed and "mora"-timed languages.

Stress timed languages (like English) are ones where the time between stressed syllables is roughly the same. Take the phrase "I went to the store with my friend John". Most native English speakers will stress "went", "store", "friend" and "John". It might not be a big difference, but you'll notice the "to the" between "went" and "store" is rushed, and that there's a sort of gap between "friend" and "John" since both are stressed. (Also, if you were to modify that slightly and say "I went to the store with my friend named John", the time between "friend" and "John" wouldn't change much at all, you'd just slip "named" into that gap.)

Many Romance languages are seen as syllable-timed, where each syllable takes the same amount of time. In French that phrase is "Je suis allé au magasin avec mon ami John", that's 14 syllables, all roughly the same timing. In Spanish it's "Fui a la tienda con mi amigo John", 12 syllables. Unless you're really drawing attention to one of the words, every syllable there gets roughly the same timing.

Japanese is mora timed, which is pretty similar to being syllable timed, except that when you encounter double-letters they double the length of the syllable. So, "Just a moment please" is "Chottomatte kudasai", where the syllables with double-t letters take twice as long. The cities Tōkyō (two syllables), Ōsaka (three syllables) and Kawasaki (four syllables) all take the same amount of time to say because the "ō" symbol means that letter gets double the length of the standard "o".

The 4 most widely spoken languages in India are Hindi (way out in front with 44% of the population speaking it as a first language), followed by Bengali, Marathi and Telugu (with about 6-8% each) The first 3 are all Indo-Aryan languages, and Telugu is a Dravidian language. The 3 Indo-Aryan languages are considered to be syllable-timed and Telugu is considered to be mora-timed.

IMO, what makes Indian-inflected English seem fast is that they're adopting the syllable / mora timing from their primary language and using it in English. That means they spend less time on syllables / words that English speakers would stress and more time on the un-stressed syllables. The overall timing of what they say is probably similar, but in evening out the length of the syllables, they take time away from the syllables that other English speakers naturally slow down to stress. Since you tend to notice the stressed words more, since they're rushed it seems like the entire sentence is rushed.

I read something ( similar to this) about the maximum data transfer per second in different languages being basically the same.

Some languages with less nuance, or fewer letters/syllables have less information per syllable, but tend to speak faster, while more 'complicated' languages have more information per syllable, but tend to speak slower.

The general trend was a maximum amount of speech 'data' that could be processed by an average human brain per second.

No idea how this would relate to second languages, and how people with 'fast' languages react to speaking 'slow' ones. Would be cool to see some data/research on it. Anocdotally, a lot of people struggle to understand Indians speaking English, is that because of the accent and/or poor English (second language, don't diss them!) or because they are speaking faster than our natural language data speed?

English (for various reasons) is kinda the only common language throughout India. There isn't actually one non-English language that you can learn and be understood throughout ALL of India, (e.g. if someone from the state of Punjab goes to the state of Tamil Nadu, they likely might need to speak English to understand each other though there are always exceptions to this) so English is very commonly spoken throughout India. As with any English speaking country, the language has changed within India and Southeast Asia over time (there is regional slang/expressions/colloquialisms unique to SE Asia like calling the 'truck' or 'boot' of a car the 'dickie/dicky'). Many of the other languages spoken throughout India are more strict in their phonetics, e.g. each syllable has a specific sound and doesn't change based on the surrounding syllables. Many English speakers who learn in India likely end up using this kind of speech pattern with English as well, leading to a different cadence in pronunciation than in other regions of the world. There are times it sounds faster, but pay attention and see if you can notice if the person speaking is using more syllables or pronouncing parts of the word you might skip over in the same word, but just faster.

I work in maritime, often alongside Indian counterparts who speak both English to me and Indian to their ship mates.

Yes, they do speak Indian just as fast. Yes, the way they speak English has a lot to do with the cadence of how they speak their native language.

As far as the flow goes, I've noticed that Indian does flow better than English just listening to it, but I don't know enough of it to make that observation with any credibility.

The oddity here is English, not the languages spoken in India. It's easy to show it by comparing vowel duration in a few languages:

• Telugu (Dravidian) - short vowels are 70~90ms, long vowels 180~195ms
• Hindi (Indo-European) - vowels are 100~180ms long
• Spanish (Indo-European) - vowels are 130~150ms long (NB: I'm analysing the data for native speakers)
• Japanese (Japonic) - tables IV-V show some data for a short /a/, 70~112ms. I'd expect the long vowels to be thus around 140~220ms, if simply doubling it (Japanese is mostly moraic after all, and open vowels tend to be longer)
• English (Indo-European) - 85~420ms

So yes, your typical language spoken in India is spoken faster than English. That doesn't say much because probably most languages are spoken faster than English.

Also, keep in mind that "Indian languages" isn't that useful of a label. It's a lot like lumping together Basque, Italian, Russian, Hungarian and Maltese as "European languages" - sure, it can be done, but odds are that you won't get any meaningful conclusion out of it, you know?

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.

I think there's a vast difference in south India and north India. South Indians tend to speak a lot faster.

I'm curious even with the different ways of stressing syllables if there is also a causal link between population density and speaking speed, similar to walking speed differences measured in rural and urban environments. Like if there is some mechanism like walking quickly to get through timed crosswalks vs larger group conversations with less time to get your point across or jump in without interrupting.

Edit: I also noticed as an English only speaker when speaking with Indians who speak English and one or more Indian languages they will sometimes repeat the last sentence you say as you finish saying it as a way to jump into the conversation or indicate agreement or having something to say but I'm not sure if that is culturally Indian or more broadly Asian, etc. if anyone has some insight into that language style.

I think this is just a dialectical thing. For comparison listen to a Mexican speaking Spanish vs someone from Spain. Very different cadence.