Rean thought about activating the Time Accelerating Feature of the Soul Gem Dimensional Realm. However, he immediately gave up the idea. First, it would indeed cost a lot of resources, which Celiskeeps insisted was not necessary.
However, the main reason was Laws Comprehension. If Rean increased the time speed in the Soul Gem Dimensional Realm, it would mean he and Celis would have a lot more time than Kentucky and Roan to comprehend their own Laws.
One must not forget that their cultivations are connected. They share their Divine Origin Energy and have to Breakthrough at the same time. If one lags behind, the other three can't Breakthrough. The time acceleration was good when their group only needed Divine Origin Energy to achieve breakthroughs. Their connection would make sure their reserves of Divine Origin Energy would always stay the same.