The German school system employs centrally organized performance assessments (some of which are called “VERA”) as a way of promoting lesson development. In recent years, several German federal states introduced a computer-based performance testing system which will replace the paper-pencil testing system in the future. Scores from computer-based testing are required to be equivalent to paper-pencil testing scores so that the new testing medium does not lead to disadvantages for students. Therefore, the current study aimed at investigating the size of the mode effect and the moderating impact of students’ gender, academic achievement and mainly spoken language in everyday life. In addition, the variance of the mode effect across tasks was investigated. The study was conducted in four German federal states in 2019 using a field experimental design. The test scores of 5140 eighth-graders from 165 schools in the subject German were analysed. The results of multi-level modelling revealed that students’ test scores in the computerized version of the VERA test were significantly lower than in the paper-pencil version. Students with a lower academic achievement were more disadvantaged by the VERA computerized test. The results were inconsistent regarding the interactions between testing mode and students’ gender and mainly spoken language in everyday life. The variance of the mode effect across tasks was high. Research into different subjects and in other federal states and countries under different testing conditions might yield further evidence about the generalizability of these results.