What actually makes sheet music reading faster? I know all the notes but still decode them one at a time.

Question 2 has a specific answer: practising one piece slowly is training that piece, not reading.

After the third repetition you are no longer reading — you are recalling, with the page as a prompt. That is a good way to learn a piece and it is nearly useless for sight reading, because the skill you want is specifically the one you use on material you have never seen.

The fix is uncomfortable and simple: read new material every day, and never repeat it. Something far below your playing level, one page, once, then move on. If you can play it perfectly it is too easy; if you break down it is too hard. Aim for material where you get through it with mistakes but without stopping.

The rule that makes it work: do not stop to correct. Sight reading is the skill of continuing. Stopping to fix a note trains stopping, which is what makes it fall apart when both hands are busy.

The bottleneck is that you are reading letters instead of words, and your flashcards trained the letters harder.

Fluent readers do not identify individual notes. They see a shape — a third, a triad, a scale fragment, a familiar chord in a familiar inversion — and their hand knows it. What is on the page is the same information, but it is being consumed in chunks of five or six notes rather than one at a time. That is why they can look ahead: the current chunk is already handled.

This is exactly the reading analogy you used, and it goes further than you might think. You do not read English letter by letter and you also do not read it word by word — you take in phrases. Sheet music works the same way and the units are intervals and chords.

So flashcards did what they promise, which is faster letter identification, and letters were never the limit.