To realize cloud computing (CC), virtualization technology has been accepted as an infrastructure of Electronic Patient Records (EPR). However, it has not been sufficiently done so far to evaluate the usability of Pen-Tablet Systems (PTS) as an input device of EPR with CC environments. From our preliminary studies, we confirmed that response time retardation, which virtualization technology inherently bears, would be an influencing factor for the usability. In this study we conducted usability questionnaires for subjects who were requested to use a drawing application that produced the pseudo retardation in order to measure an allowable range on the retardation for implementing PTS on EPR with CC environments. Usability of the PTS would drastically drop with increasing values of the scattering of delays exceeding 35 milliseconds. As a result, this study would provide useful measures for implementing PTS with CC technology.