He was cultivating the Nine Dragon Universal Circulation Technique. This technique not only gave Ye Xiao a very strong body but also let him form dragons.
After he created the first dragon, the Heaven Devouring Divine Dragon, he gained the ability to devour, and simultaneously, he also got an invincible foundation where no matter how much he devours or increases his strength by other means, his foundation will remain stable and strong.
But... Zhao Yufei and Zhao Qing'er were not Ye Xiao. The moment they made continuous breakthroughs, they felt their foundation weakening.
Ye Xiao wanted to let them increase their strength continuously but after seeing that their foundation started has to weaken, he decided to stop for a while and let the two girls stable their foundation. After that, he will once again give them the Water Spirit Herb and help them to increase their cultivation.