Different languages don't actually have different rates of information density.
Poor choice of words perhaps, but some sentences simply take slightly longer to parse in different languages.
That's not true, either, not in any meaningful way. Native speakers do not have any more trouble parsing language A than they do language B. It does not take them any more time.
Err... this is strictly untrue. There are most definitely languages that have more information density by a great many metrics (at least on average), and are certainly pros and cons of different aspects of different languages in regards to both "easy to learn" and "easy to parse" and "information dense" and "unambiguous to interpret". A lot of language drift is caused by a desire among a subpopulation to change one or more of these aspects.
Now, for a native speaker, this will be almost unnoticeable because human language processing can get, like, super optimized, but there are definitely languages where you can communicate the same thing a lot more quickly or a lot more precisely than in others (on average, between speakers of equal skill).
I think it's also important to make it really damn clear that the written and spoken (and other) "versions" of a language are in many ways different languages entirely for every consideration of the points above.
Yes, and
when we talk about aspects of a language that are not strictly written, we mean the spoken language. It's a well-known law of linguistics that human languages all have about the same rate of information density (For the record, this means information per unit of time, not per unit of sound- languages with fewer phonemes tend to be spoken more quickly, but the rate of information transfer is the same). Furthermore, language "drift" has very little to do with information density- I advise that you skim William Labov's
Principles of Linguistic Change. Sound and morphosyntactic change is unconscious, and has much more to do with sociolinguistics than it does with information density. (It is true, of course, that sound change can obscure previous distinctions. In this case, speakers simply innovate new forms and structures to compensate; see Southern American English's innovation of "stick pin" and "writing pin" to cover for the pin-pen merger. But the innovation of such structures indicates that human languages do
not significantly differ in information transfer rate- if they were, this wouldn't happen so consistently.) It's worth noting that attempts to increase information density by engineering "more efficient" languages have met with failure because the human brain can't handle them (see Ikthuil).
I think part of the problem here is that English speakers, who use a relatively analytic language natively, have a hard time internalizing large inflection patterns; and based on our understanding of "word", that means we conclude that more inflected languages have a higher rate of information density. But that's simply not true;
hylly-i-llä and
on the shelves (save for the article, which isn't relevant- it's governed by pragmatics and syntax more than semantics; and in any case, English has no analogue of the vowel harmony at work in the Finnish example) have the same number of morphemes, and they don't cause their native speakers any more trouble than the other. It's not that you're wrong- really you're not even wrong. What's your definition of "metric," for example?