I'm not to sure if I'm correct here but if you have a 100Mbps router and a 10Mbps download speed from off your ISP, you'll only be able to download no quicker than 10Mbps over the internet, however, with your router being 100Mbps, you can transfer files, computer to computer on your LAN at high speeds of 100Mbps, but that will also mean having a NIC on your computer that can handle speeds like that in order to transfer files at 100Mbps.