In mathematics, deliberate practice amounts to working out a high volume of practice problems at the edge of your ability, with proper scaffolding and rapid feedback, and continually pushing forward into more advanced problems.

You don't have to be blazing fast but you do need to have sufficiently tight feedback loops so that you can get through a high volume of action-feedback-adjustment cycles. It's the compounding of a massive number of those cycles that produces a massive gain in performance.

Also, in order to make mathematical discoveries -- or more generally, in order to grapple with advanced content at all -- you need to develop a level of automaticity on your foundational skills.