That would probably range from something that's so easy you just want to drop the hammer and knock it out to a massive problem that you'll spend months or years chipping away at. Either way if you really are interested in working on it you'll think about it in the background and approach solutions from multiple angles. When you're excited about working on the problem you'll have no issues with working more than 10 hours per week if that's what it takes.
Everybody is different but I've found that when a problem is actually fun to work on "productivity" techniques don't really do much. It's mostly making a plan for what to do, working a bit, learning more, and either solving or starting the work cycle over again with new information.
So I think you probably need to work on more problems that you're actually interested in.