This study compared the effects of extracurricular synchronous computer-mediated communication (SCMC) and asynchronous computer-mediated communication (ASCMC) between students and teachers on students' digital reading performance at different frequencies. 392,269 samples from 53 countries/regions that participated in the Programme for International Student Assessment 2018 were collected. Multilevel regression analysis showed that SCMC negatively influenced digital reading performance across countries/regions. As the frequency decreased, the negative effect of SCMC diminished. In contrast, ASCMC at a moderately low frequency could facilitate digital reading performance in some countries/regions; however, as frequency increased, the positive effect became negative. These results showed that synchronicity played a role in predicting students' digital reading performance. This study also explored the mediating effect of metacognition with Nelson and Naren's metacognitive control-monitoring model. A multilevel mediation analysis proved that the effects of SCMC and ASCMC on digital reading performance were mediated by students' metacognition of assessing credibility. Practical implications and suggestions for students' self-paced learning were discussed with the purpose of promoting the effective use of extracurricular CMC between students and teachers and improving students' digital reading achievement in the post-COVID-19 pandemic era.