The goal of the present study was to address the analytical complexity of incorporating responses and response times through applying the Jeon and De Boeck mixture item response theory model in Mplus 8.7. Using both simulated and real data, we attempt to identify subgroups of responders that are rapid guessers or engage knowledge retrieval strategies. When applying the mixture model to a measure of contextual error in linguistics results pointed to the presence of a knowledge retrieval strategy. That is, a participant either knows the content (morphology, grammar rules) and can identify the error, or lacks the requisite knowledge and cannot benefit from spending more time on an item. In contrast, as item difficulty progressed, the high-ability group utilized the additional time to make informed guesses. The methodology is illustrated using annotated code in Mplus 8.7.