Math is dense . The flow of a math book is not like the flow of a novel. Reading a mathematics textbook requires slow and careful reading of each word. A typical novel might be read at the rate of a page a minute. Expect to spend 30-60 minutes working through the few selected pages for each reading assignment thoroughly for the first time.
Every word counts.
Writers of math texts believe that extra words and repetition get in the way of clarity, so there is little chance of picking up missed information from reading on. Expect to re-read. And then to even re-read it again. It will take several passes through a section before you start to absorb the material. Do not skim diagrams and other kinds of illustrative material.