Based on my own personal experience, knowing full F2L, OLL and PLL can get you to sub 10 seconds without extreme difficulty. I do not really focus on extra cases/algorithms nor X-Cross, but rather focus on finding the next step before I finish the current step. In other words before I finish the cross, I try to find the first pair, before I finish the first pair, I try to find the second pair etc. I do however suggest practising other methods to widen your knowledge/feeling of the cube. Other methods such as learning blindfold, ZZ, or roux in particular. Concepts such as freestyle block building, edge orientation, commutators will be useful for any serious speed cuber.