Speed

The reason he was particularly fixated on reducing the combat speed gap between him and his opponent was that he did not have faith in his passive defensive capabilities. If the speed gap between them was high, then that meant he would not be able to evade attacks, which naturally meant he would be forced to endure the attacks with his body.

Given that the attacks were naturally fast enough to bypass his ordinary reaction time, he did not possess the ability to defend against them with active defensive techniques. Active defensive techniques like Inner Divergence and Flux Earther require being able to intercept attacks to some degree, which he could not do if he couldn't react to the attacks due to them being too fast for him.

This meant that if he could not react to the attack then he could only rely on passive defensive techniques such as his Adamant Reforging where his body was conditioned to be tougher generally against all forms of damage.