Reading fiction and non-fiction (I assume this is what you mean by 'hard) are effectively different activities.

Reading fiction is about getting caught up in an imagined world. The stakes are lower and the expectations are lower. This makes it easier. If you miss the paragraph where a character is described as having a beard, you won’t feel the need to go back to reread when you find out they do four chapters later when they scratch said beard.

Reading non-fiction is about building new conceptual structures in your mind. It requires more attention. If you fail to understand Nietzsche’s ressentiment in ‘Geneology of Morals,’ you are unlikely to be able to understand the later concept of the ‘priestly’ morality. You will not be able to fully construct the conceptual structures the book attempts to generate.

Because of this, reading non-fiction is a process of doing whatever is necessary to get all the parts and relations into one’s mind in order to produce a mental tool and then, hopefully, learn to use it. If notes help, write them. Rubber ducking can help. Simple repetition can help. Slowing down can help. Talking to other readers (as in a study group (shoutout to @DGQC on YT)) can help. Concept mapping can help. Sleeping on it can help. Highlighting can help. Cross referencing can help.

It’s study. What helped you learn in school? Do that.