I think my first line of attack would be to connect a single PC to the Cable Modem's ethernet port and see what sort of performance you get with that.
If that works well, reconnect the router to the Cable Modem and then connect the *single* PC to the router and again gauge its performance.
If again that works well, try connecting each of the seven computers in turn to see if one of them doesn't perform well. A "problem" computer might be having an effect on the network as a whole.
You haven't said how you are actually connecting the seven computers because most routers have only a four-port network switch built in. If you are connecting "wirelessly" with seven computers, the 54Mbps bandwidth (which in practice will be nearer 30Mbps) will be *shared* between the connections, each computer will not have use of the full bandwidth.