After the group entered the desert, they realised it wasn't as dangerous as they previously thought it would be. The creatures in the desert were just a little stronger than the second floor. It was easy to kill them; therefore, Ayaan decided to take the full advantage of the situation. At first, he injured the creatures and let Noor or Isobel kill the creature.
In just a short period of time, Noor was able to reach the high mage after killing a few high mages. Though the benefits weren't as visible in Isobel's case as in Noor's. It was still able to boost her strength.
After spending more than two hours, the group was able to reach in front of the wall on the fourth floor. It wasn't that he couldn't clear the floor faster but he deliberately slowed his speed down because he wanted to increase Isobel and Noor's strength.