Textbooks currently include many elaborations that describe, illustrate, and explain main ideas, increasing the length of these textbook chapters. The current study investigated if the cost in additional reading time that these elaborations impose is outweighed by benefits to memory for main ideas. Given that elaborations in textbooks sometimes fail to produce memory benefits, the current study also investigated if the reason is that less time is spent reading main ideas sentences in elaborated versus unelaborated texts. In two experiments, participants read a textbook passage with just the main ideas or with these main ideas and elaborations. Two days later, participants completed tests of their memory for the main ideas. Conceptually replicating previous research, elaborations did not provide a memory benefit commensurate with the time cost they imposed. Results also indicated that the lack of benefit is at least partially attributable to less time spent reading main ideas for the elaborated versus unelaborated text. To further investigate why students spent less time on main idea sentences, Experiment 2 provided evidence that this difference may be due to difficulty discriminating main ideas from elaborations while reading. In sum, textbook elaborations may impair memory for main ideas due to less time spent on these main ideas despite the large overall time cost imposed; thus elaborated texts can be less effective than unelaborated texts.