The True Demon Technique had a total of five different stages.
The Scarlet Demon, Bane of the Living, Murdering Asura, Death Reaper, and finally, the True Demon.
Each of these stages represented one particular part in the many realms of cultivation.
The Scarlet Demon Stage represented the six realms of Martial Cultivation.
The cultivator in this stage other than growing increasing their strength by using the cultivation technique, they can grow at an even faster rate by killing humans or staying near recently killed humans and absorbing the Yin Qi which would be released upon their death.
Unfortunately to achieve this condition, Bai Long needed to kill more than one individual at the same time.
Since this never happened even once, Bai Long was unable to increase the speed of his cultivation inside the Martial Realm.
Not that he ever needed to anyway.