Good reading is not necessarily fast reading. Good reading isn't reading that you only have to do once to "get it," either. Good reading is reading that gets what you need done done.
When readers interact with texts—that is, when they read—they create meaning in their own minds, meaning that is made through the interaction of what the writer has put into the text and what the reader already knows, believes, or wonders about. Reading is part of a three-part triangle formed by the writer, what he or she writes (the text), and the reader.
Different readers will make different meanings even when they read the same thing (one reason why it is such a good idea to form reading discussion groups!) You are a good reader if you can make meaning out of something that you read, and if that then helps you to do whatever it is you need to do—answer test questions, respond to the reading in writing, create an argument.
This may sound a lot different from what your experience with reading has been—making meaning sounds more active and participatory than reading word after word and hoping that it begins to make sense, perhaps feeling bored, tired, or perplexed. Or it may be that you already understand that good reading is active, but not necessarily fast.