If this is a central tool in the employees' daily workflow, my approach would be to test both solutions on a number of people, probably around 5 - 6 subjects per solution.
In terms of determining which is faster, this will be as straightforward as starting and stopping a stopwatch, but depending on the fidelity of your prototype, conducting a formal test would allow you to identify usability pros and cons to either solution that are maybe not as obvious as time spent. For example, imagine the single-page solution is 30 seconds faster on average, but the test users generally express annoyance with a loss of overview. Would this be justified by a relatively small temporal advantage?
This conclusion is made from the assumption that this is a tool that will be used often by the employees. If this is indeed the case, overall UX should be taken very seriously, and conclusions shouldn't be made on assumptions of prior knowledge.