Learning to process large quantities of material and make sense of it is one of the important skills to acquire during your studies at university. With practice you will be able to increase the speed with which you read, learning effective techniques to extract the significant points and arguments within a book or article. You should be able to get the 'gist' of a text in a relatively rapid first reading, though you will almost certainly need to read prescribed reading and the main course materials more than once to absorb the most important points, understand them fully, and focus on the relevant detail. The best approaches to reading effectively may vary according to what you are reading, and what you want to achieve in your reading.
To gain an understanding of what you are reading you might, for example, first scan the text quickly to get a broad overview of what it contains. Then read it again more slowly, picking out the main ideas and how they are developed. Finally, read it again in detail.
You should aim to read with attention and comprehension, making sure you understand all the important concepts, at the same time, carefully evaluating the material in the light of what you already know. This is the stage when you may find it helpful to highlight (if you own the textbook) the more important ideas in the text, or to make your own summary notes of the key points.
One useful timesaving tip is to check the contents page or index of a textbook and to look at headings and sub-headings as indications of important points or concepts or developments of an argument. Consider what you are reading in the light of what you already know and have a few questions at the outset. When you read the text for the second time note down or highlight the main points and try to recall what you have read in your own words. Check your understanding by looking at the text.