If you have heard of K. Anders Ericsson or deliberate practice you’ll know this has limits. The fastest way to get great at anything when you’re already good is not play. It’s deliberate practice. You work at the edge of your ability, doing something you are just barely capable of doing, trying to get to competent. Once you’re competent you move on to the next frustrating thing that you can’t quite do reliably. It’s closer to mentally exhausting than fun.

That's the fastest way to grow, but to retain that growth it's also worthwhile to have some 'fun' with each new skill shortly after becoming competent at it and periodically (say, monthly at a minimum) thereafter. At least, that's how it is in my experience with shred guitar techniques. The higher level stuff gets rusty faster than the fundamentals