I've found that there's generally two levels of paper understanding. The first is achievable in a day, and consists of recognizing what the authors did and roughly how their technique works and why it's different. The second, much more nuanced level of understanding comes when you really dig into the paper and analyze every sentence, and commonly occurs when you need to extend or implement it. The writing is rarely wrong, but many things that are not relevant are either easier said than done or painful but boring implementation details. At that point, you could rewrite their paper from scratch if needed.