Students’ graph understanding was chosen for the research because teachers, especially physics teachers, tend to use graphs as a second language, assuming that their students can extract most of the information from them. This research aims to determine the differences between Serbian and North Macedonian students’ graph understanding of kinematics and their perceived mental effort. Differences in physics curricula in the Republic of Serbia and Republic of North Macedonia are taken into account and compared as well as students’ achievement on the TUG-K knowledge test to find explanations of potential differences and guidelines for change in the approaches to physics teaching. The sample includes 630 first-grade grammar school students (313 students from Serbia and 317 from North Macedonia) from randomly selected classes. Mann–Whitney U-test determined possible differences in student achievement and perceived mental effort. The main results indicate a difference in students perceived mental effort. North Macedonian students perceived less mental effort than Serbian ones, which leads to higher instructional efficiency of teaching approaches applied in North Macedonia than in Serbia. Based on the results, the recommendation for improving the Serbian education system lies in reducing mental effort through efficient allocation of teaching hours.