This. Most grad school classes are poorly taught and the professors indifferent or discouraging to actually helping you learn. PhD students are assumed to be capable of learning these things on their own or already knowing them. If you are encountering things for the first time, you'll likely be behind.

In contrast, if you come in mostly ready to go and these classes are just refreshers, you can spend time in that class working on actual research and impressing the prof as well as not panicking if/when you realize you don't understand what's going on.