Understanding takes longer than memorization because it has to survive unfamiliar examples.

Memorised material only has to survive being repeated. Understanding has to survive being applied to a case you have not seen before, which is a much harder test and takes correspondingly longer to build. That is why people who learned quickly often collapse at the first unfamiliar problem.

Test yourself with variations rather than repetitions. Take what you learned and apply it to a situation that was not in the material. Where it breaks, you have found the boundary of your actual understanding, and that is where the next bit of work belongs.