Understanding learning differences is crucial for education; however, quantitative evidence describing learning rate patterns is scarce and faces unaddressed biases. This paper examines the variability in Brazilian students’ individual learning rate as they work to improve their argumentative essay writing skills for a high-stakes national assessment to access tertiary education. 12th-grade students engage with a personalized learning platform, providing them with multiple practice opportunities to submit essays and receive personalized feedback. I estimate individual learning rates using multilevel mixed-effects methods and analyze the estimates for both total writing scores and the separate components of the essay writing domain. This study contributes to the learning differences literature by offering novel quantitative evidence on individual learning rates heterogeneity in open writing tasks. The findings support the importance of individualized educational solutions to reduce learning gaps by optimizing the pace of deliberate practice opportunities offered to the students.