Nasz koń je powoli / Nasz koń powoli je - Our horse eats/Our horse is eating slowly
Nasz koń MA POZWOLENIE na jedzenie powoli/ WOLNO mu jeść powoli/ WOLNO mu powoli jeść - Our horse HAS PERMISSION/FREEDOM to eat slowly/ It IS ALLOWED to eat slowly/ Our horse IS FREE to eat slowly
They are similar. I would say that "powoli" is often used when you ask someone to slow down (also "wolniej" - slower) or when someone does something slowly and precisely/thoroughly. On the other hand, "wolno" is often used when something is being done too slowly. But generally, they are synonyms.