Why can I understand slow English but not fast English?

Because slow English and fast English are two different codes, and only one of them is in your memory. Careful speech delivers each word in the form you learned, sounds separated; natural speech regroups those sounds into syllables that ignore word boundaries, so a familiar word arrives in a shape you have never stored. The vocabulary is not the problem and neither is intelligence; the mapping is. Learn the four rules, then practise them on marked transcripts until the joins are audible, which is a different exercise from hearing more audio.

Fast speech is heard in chunks of several words at a time, so practising word by word trains the wrong unit. A test you can run on one sentence: take a line you already understand at slow speed, play it three times at normal speed, and write down only the words you are sure of. Being sure of fewer than half points at decoding rather than vocabulary, and no amount of new words will move it. Repeat that same sentence daily for a week and watch whether the count of words you are sure of rises; the count, not a feeling of speed, is the progress signal.

Why Fast English Sounds Like One Long Word: ielts listening tips for band 9 →