Definitely keep all workstations wired - even top-speed wireless is usually several times slower than wired.
Beyond that, if there's an anti-virus, compare the performance with that on vs. off.
Beyond that, do a test comparison with the computer connected to the network switch/hub with a different network cable - or even to a different switch if you have more than one.
You can give us an idea of the network connection speed by starting VJS, choosing Options at the top of the initial File Center screen then select About, then in the box at the bottom of the About screen type 20000 and <Enter> and let us know the results - something like
