Is a cast iron wok too heavy?
That will be extremely heavy! Very cumbersome.
Traditionally, woks are not cast iron, but steel. They get really hot really fast. They are easy to flip stuff around in as you stir fry. Get one with a flat bottom so that it has maximum contact with your heat source as our stoves don't get anywhere close to as hot as a traditional heat source for a round bottom wok.
America's Test Kitchen does a wonderful video on the subject!