Mobile Personal Health Records (mPHRs), which make it possible to track and manage users' health information, can be an important aid in improving people's health. Despite its potential benefits, poor usability of systems can hinder the adoption and use of mPHRs. This study aims to evaluate the usability of a mobile health application in terms of perceived cognitive workload and performance. The cognitive workload experienced by 30 volunteers (15 experienced and 15 inexperienced), was measured while performing the given tasks with the NASA-Task Load Index (NASA-RTLX) scale, and the duration of the fulfillment of the tasks by eye tracking device. While there was no significant difference between the two user groups in the completion time of the tasks, a significant difference was found in the perceived cognitive load. "Making an appointment", which could take much longer to complete than other tasks, resulted in the highest cognitive load for all users. Further usability research using think-aloud protocols and user interviews could provide insights into design improvements for reducing cognitive load and enhancing performance.