An adequate level of reading comprehension is a prerequisite for successful learning. Numerous studies have shown that without a solid foundation, there can be severe difficulties in later learning and that failure in the first years of schooling can determine attitudes to learning. In the present study, we present the effect size of an online game-based training program implemented on eDia. The primary goals of the development program are to develop fluency in reading and reading comprehension in Grades 3–4. The content of the program has been developed in accordance with the national core curriculum and the textbooks based on it. Therefore, it can be integrated into both classroom-based lessons and extracurricular activities outside of class. The quasiexperimental research involved 276 students. Propensity score matching was used in examining the effect size of the development program to increase the validity of the results. Through the training program, the development of students in the intervention group accelerated greatly (d = .51), which proved to be even higher in the lowest and average skill groups (d1 = 1.81; d2 = .92) as well as in the disadvantaged student group (d = .72). Latent-change analyses confirmed the sensitivity, relevance, and importance of developing comprehension at 9–10 years of age and the generalizability of the results (χ2 = 421.5; df = 272; p < .05; CFI = .950; TLI = .945; RMSEA = .045 (CI: .036, .153). The study provided evidence that a well-designed online training program is suitable for developing comprehension and overcoming disadvantages, even without the presence of the teacher outside the classroom.