I feel they only explained the "what" but not the "why". i.e., you do this and this and suddenly your code is 250x faster. But I want to know _why_ this is fast

I feel they only explained the "what" but not the "why". i.e., you do this and this and suddenly your code is 250x faster. But I want to know _why_ this is faster, or _why_ the old version is slow, which is not explained.