You re-read sentences much more often than you might suspect and it happens with all kinds of sentences, even grammatically correct ones. Garden-path sentences, for example “the old men the boat”, are specifically crafted to demonstrate this and they essentially are doing the same thing as using ‘r’ and ‘u’ as a substitute for words: they violate the Gricean maxim of manner and that one relies a lot on expectation.

However, one could make a case that in some situations, like a “how r u” via text, the replacement is ubiquitous and somewhat expected and doesn’t cause any impediment to understanding. It’s definitely a hinderance when a more verbose communication is expected. Might be a neat subject for a phonetic study, honestly.