When Kagami Uchiha left the mental space, little time had passed in reality. Genjutsu worked by influencing the chakra in the brain. Although chakra gave it certain extraordinary properties, its physical effects still remained.
Because the brain governed the perception of time, time within a genjutsu world did not necessarily match reality. The more advanced the genjutsu, the more capable it was of achieving this. Uchiha Itachi's genjutsu, Tsukuyomi, was one example: even after three days and nights within its world, only an instant would have passed in reality.
Kagami Uchiha immediately reverted his Mangekyō Sharingan to ordinary Sharingan. Although little time had passed in reality, the time spent in the genjutsu world had still been sustained by his Mangekyō Sharingan, consuming a tremendous amount of chakra.
The Mangekyō Sharingan possessed immense power, but perhaps that power was too great even for the Uchiha. Even the bodies best suited to the Sharingan still felt the strain of using it. Madara had been utterly exhausted the first time he used his Mangekyō Sharingan, and only gradually improved as his chakra increased and his body matured.
Kagami Uchiha was already in a shinobi's prime. Apart from the discomfort of using his Mangekyō Sharingan for the first time, he had adjusted considerably. By comparison, the younger Uchiha Ken probably felt its physical burden more readily.
"Is it over?" Roshi came to his senses. When the power of the Mangekyō Sharingan invaded his mental world, everything had seemed to spin around him. He had no idea how much time had passed. By the time he felt better, Kagami Uchiha had closed his eyes.
"Yes." Kagami Uchiha nodded slightly. "Thank you for cooperating."
"I should thank you too. I was worried you might use your Mangekyō Sharingan to probe for intelligence or control me," Roshi