Yōga Sūtra 1.22

मृदुमध्याधिमात्रत्वात्ततोऽपि विशेषः।

mr̥dumadhyādhimātratvāttatō'pi viśēṣaḥ।

[मृदु मध्य अधिमात्रत्वात् ततः अपि विशेषः।
mr̥du = mild; madhya = medium; adhimātratvāt = extreme; tataḥ = they are; api = as such; viśēṣaḥ = distiguished;]

As such they are distinguished as mild, medium, and superior.

Based on the intensity and speed of practice the practice is classified as mild, medium, or extreme.