The Mass Heal of the dervishes is not improved via the herbal medicine upgrade.
Herbal Medicine states that it increases the healing rate of religious units by 60%. Dervish is a religious unit and mass heal does in fact heal. So one would assume the +60% would apply to mass heal but it currently does not.
I tested it using logistics wing and healing four castle age ghulams from 19 health to max using only the dervish aura. Two without herbal medicine, two with herbal medicine: In all cases it took exactly 1 minute 30s - so the herbal medicine upgrade does not apply to dervish mass heal.
So even though the dervish is a religious unit, the (mass) heal is not increased by +60%.