Mrdu soft, feeble, mild, fickle
Madhya middle, intermediate, moderate, average
Adhimatratvat ardent, steady minded, keen
Tatah thence, further
There are differences between those who are mild, average and keen in their practices.
Sadhakas are of different levels of eagerness and intensity. For them, the goal is time-bound, depending upon their level.
These types can be further sub-divided. For instance, a keen sadhaka can be feebly, moderately or intensely keen. Similar subdivisions can be made of the average and faint kinds. The goal of yoga is near or far, depending upon one's eagerness and one's efforts.
This sutra pertains to the various aptitudes of yogic practicians. But if sutras 1.14-22 are studied as a group, it is clear that they refer to nine types of yogis who are highly evolved and whose parameters of intelligence are far beyond ordinary human standards. They are of an ascending order of intensity.
Paravairagya (supreme detachment) is for those who are clear of head and pure of heart, valiant and extremely energetic (adhimatrataman or uvra samvegin). For these, the goal is imminent; for others, it is time-bound.