The beat of the sentence: which words carry it and which almost vanish
Content words carry the rhythm of spoken English; the grammar words between them shrink to fit.
I've been waiting for a bus for half an hour.
Spoken English does not give every word equal weight. In the model sentence, only four words are pronounced in full: the verb, the noun that follows it, and the two words that measure the time at the end. These are the content words - they carry the information, and they carry the beat. Everything between them - the auxiliaries, the articles, the two identical prepositions - is squeezed into brief unstressed syllables, most reduced to the neutral vowel. The stressed words arrive at roughly even intervals, and the grammar words compress to fit the space between beats. This is not lazy or casual speech: it is the normal rhythm of the language at every level of formality.
Learners taught from the written page give every word its full dictionary pronunciation, and the damage runs in both directions. Spoken that way, a sentence sounds mechanical, because the rhythm a native listener uses to locate the important words is flattened away. Worse, the same habit ruins listening: if you expect the auxiliaries and prepositions to sound as they do in isolation, you will listen for full forms that never arrive, decide that native speakers swallow most of their words, and blame the speed. The speed is not the problem. What sounds like missing words is the set of weak forms - and once you listen for a compressed neutral vowel instead of the citation form, whole stretches of natural speech become audible.