It does indeed seem like the arrow of causality may be backwards here. Many people I know who are truly naturally talented are relaxed because their work does not subjectively feel difficult to them.

Yes, they push themselves hard and strive constantly to improve. But when the nature of your work matches the contours of your mind perfectly, it's not really work – it's purpose. In this zone you can feel yourself excelling and striving with a sort of effortless ease. You needn't force yourself to study - on some level you want to sit down and practice. The feeling of being truly excellent is addicting and propels a virtuous cycle of growth and achievement. So yeah, of course you're relaxed. Winning feels great.