Within the context you've described, I would agree that it would be Department, as it is involved in the filter for the list results. Presumably you would want to expose this and index the resulting column for optimal performance.
However, any additional factors you have not stated could have an impact on the overall performance, like enabling column sorting and/or filtering on the grid that is displaying the results.
I'm not sure what the context of your question is. Why do you want to know which un-optimized property would have the greatest performance impact?
This was actually a question asked in the mock exam. We are conflicted on the answer, mainly because we come from .NET background and seems like Pega has a different concept on non-optimized/un-indexed fields