Your multicast/broadcast could be the issue. Go to the VLAN that your users are on, and Enable BCMC Optimization to suppress multicasts and broadcasts: Go to configuration> Network> IP. Edit the VLAN number that your users are on and Enable the checkbox next to Enable BCMC Optimization, and click on apply in the lower right hand corner. Remember, if you have any applications that depend on broadcasts or multicasts, they will not work when you do this.