Firstly he is much much older now, we notice it more as he stopped then started. If he was wrestling consistently over last 8 yrs and getting bigger and changing his style we wouldn't notice as much.
The rock is so much more of a unit now for his films he's not nessecarily slower in efforts more to the fact mobility will be a lot more restricted to lack of range of elasticity.
The more fat u lose great but muscle weight is stupidly heavy. He could easily cut up and keep size and improve cardio but he would never be as quick as hea too heavy. Also remember once you hit 30 your body reacts very differently to exercise and recovery/ improvig body you slow down.
I have been working out 10 yrs and im only 28 and i notice a difference over last few yrs already. Im afraid to say no matter what we will never see the rock of old again. Angle hhh xpac hardyz etc anyone fast etc never again.