Incredibly enough, I think you're underestimating Federer's abilities on clay. Even there he used to spend not too much time on the court (when he was playing well), so now that his level has come up again that could well be the case. Also, he has traded speed for placement in his serve, which is something always good on clay, and also has stepped up the aggression on the ROS (for better or worst, this will shorten points). Off course clay matches will on average last longer than the HC ones, but it is not like every match will be a long three hour grind...
*(just edited as last night I fell asleep while typing, and somehow managed to hit the post button with an incomplete message, just don't ask me how...)