The Sacred Heart Art was powerful because it was a cultivation technique that let cultivators keep growing stronger as long as they lived. It just wasn't suitable for Ling Tian and the others to practice right now. After all, they didn't have much time to spare, and this technique would consume a great deal of their mental energy. They would be better off using that energy to strengthen their newly opened Small Worlds; that would do more to improve their strength.
Of course, even if Ling Tian and the others couldn't practice it now, they still could after rescuing Hua Min'er and their other friends and family. By then, they would have plenty of time and long lives ahead of them. Assuming nothing unexpected happened, they would have already obtained the Creation Divine Tree by then, and wouldn't need to worry about lacking life force.
For now, what mattered most to Ling Tian and the others was to work hard to improve their cultivation and strengthen their opened Small Worlds. That would do more to increase their strength.
Ling Tian stopped dwelling on the Sacred Heart Art and focused on improving his strength.
Time slipped by. In the blink of an eye, another ten or twenty thousand years had passed. Their cultivation had slowed somewhat, since they had not undergone another tempering through opening Small Worlds, but that was only by comparison. After all this time, their realms had still advanced considerably. Most of them had risen by one or two minor stages, Ling Tian included.
That was right. After nearly twenty thousand years, Ling Tian had broken through again, reaching the peak of the Twenty-fourth Heaven of Near-Saint. His next step was the Twenty-fifth Heaven of Near-Saint, and once he broke through, his strength would undergo a qualitative leap.