The speed of learning isn’t linear.
It took me a long time to memorize the changes to play my first jazz standard from memory. It was probably the F Blues, and it must have taken me hours of work with my limited piano skills at the time.
The second one was a little faster, and the third one faster still. These days, I can memorize a tune relatively quickly and be able to play it passably from memory to accompany someone playing or singing.
What changed? It wasn’t the knowledge, it was the meta-learning. With each successive song, I acquired two things:
First, I learned my process. I learned what worked for me to memorize quickly.
Second, I developed a pattern library. I absorbed the similarities between them, the patterns that appear over and over again. With that, I was able to memorize in chunks rather than chord-by-chord. It’s a little like the fact that learning your tenth word in Italian is a lot harder than learning your thousandth. By 1,000 words, you’ve dealt with lots of nouns, lots of verbs, lots of adjectives – so your brain knows what to do with the new one…”I’ll put it next to these others. Easy.”
The early stages of learning are hard, because you don’t have that process, you don’t have a pattern library. And more often than not, you don’t know that you will acquire either of those – you’re working blind.