Cognitive Load Theory
Your working memory holds about four things at once — overload it and learning just doesn't happen.
Grounded in the research on cognitive load theory (john sweller, 1988)
the bottleneck
Working memory is the mental scratchpad you think with right now, and it's tiny. Miller's 1956 '7 ± 2' was the famous number; Cowan later revised it down to about 4 chunks. John Sweller (1988) built cognitive load theory on that limit: blow past it and new info falls on the floor before it sticks.
intrinsic load
The difficulty baked into the thing itself. One chord vs. sight-reading a whole song — the second is just more interacting parts to hold at once. Baddeley's model says you've got separate small slots for words and visuals. You can't delete intrinsic load, only break it into pieces and stack them in order.