One should understand that not all processes in life take 45 minutes. Learning meaningful content is like going to the gym or playing the piano - you cannot expect to see any meaningful results in a day. You'll have to put in the work for a very long time, but you'll get better at it as you go.

Analogy aside, not all text is written to be read in a 45 min session. Set your mind that reading and absorbing content from a text book can take anywhere between 3 months to 3 years. Some concepts may take decades to really click in your mind. Luckily, the content in text books is usually broken down in chapters and sections for you, so consider each of those a "less text" content, and read "less text" every day. Set a goal of understanding a single section or chapter per day/week. At the end of a year you'll have read "long text" without ever reading much in a single reading session.

Research papers can be read in anywhere from minutes to a couple weeks. But if you are new to the field, you may have to read them over and over, and that time can extend to months until you understand the implications of what is written.

Don't rush it.