Według mnie są to synonimy. :) Częściej słyszy się jednak "wolny". (np. Ten samochód jest strasznie wolny) Słowo wolny odnosi się też do "wolności", "bycia wolnym" - powolny w tym kontekscie już nie użyjemy (np. Wyszedł z więzienia i teraz jest wolny)

Ten samolot jest bardzo powolny. = The plane operates slowly. (= as in it could fly fast no problem, but due to some reason it's flying slowly, hinting at an abnormality in operation) Ten samolot jest bardzo wolny. = This plane is very slow. (As in the structure of the engines etc makes it slow in general, it's working properly, it's just slow by design) From my understanding, in relation to speed, I'd use "powolny" to describe an action, hinting at a possibility of faster operation - like a slowly working waiter (=powolny kelner) a slowly working construction worker (=powolny budowlaniec), a slow paced walk (=powolny spacer). "Wolny" means firstly "free", despite it meaning "slow" as well, I try to steer clear of using it in that context, to avoid possible miscommunication. Observe: (At a taxi stop) - Wolny Pan jest? =Are you free (to ride with)? / =Are you slow? - Nie, szybki. (=No, I'm fast).