Actually, in this scenario, a hub would actually be faster by perhaps a millisecond or two. A switch simply provides microsegmentation to avoid collisions, and gives dedicated bandwidth to seperate connections. However, this only benefits you if you have two or more separate connections. For example, if you had three PCs and the internet, and pc1 was transferring a file to pc2, while pc3 was downloading something from the internet. pc1 and pc2 would have a full bandwidth connection available between them, as would pc3 and the internet, their traffic couldn’t cause collisions. However, if the only connection the majority of the time is to the same place, this situation being the internet, the switch would only add in a bit of latency while not really providing any benefit.
The router has a hub built into it. Normally, a simple router would be just two ports, one for the internet and one for you local network. You would then uplink that local port to a hub or switch. However, to simplify things, almost all home broadband routers have built in hubs or switches. You can utillize this built in hub, however you must turn off DHCP like Marty_D said. To do this, you’ll have to log into the router’s web based control panel and disable it. Connecting to the control panel should be in the installation manual. To then utilize the hub, just treat the LAN ports like a normal hub, just forget the single WAN port even exists.
The simplest solution though, and cheapest, would be to just take back the router and get a hub or switch, if possible.