Keystroke logs provide a comprehensive record of observable writing processes. Previous studies examining the keystroke logs of young L1 English writers performing experimental writing tasks have identified writing processes features predictive of the quality of responses. Contrarily, large-scale studies on the dynamic and temporal nature of L2 writing process are scarce, especially in an assessment setting. This study utilized the keystroke logs of adult English as a foreign language (EFL) learners responding to assessment tasks to examine the usefulness of the process features in this new context. We evaluated the features in terms of stability, explored factor structures for their correlations, and constructed models to predict response quality. The results showed that most of the process features were stable and that their correlations could be efficiently represented with a five-factor structure. Moreover, we observed improved response quality prediction over a baseline by up to 48%. These findings have implications for the evaluation and understanding of writing process features and for the substantive understanding of writing processes under assessment conditions.