Devices on the gigabit switch will be able to talk with each other at 1Gb/s network speed (of course, system throughput will be less), while devices on the router will talk with each other at 100Mb/s or 10Mb/s as appropriate. The router's LAN to WAN throughput will already be on the order of a few Mb/s, and so the fact that you have to drop to 100Mb/s on the link from the gigabit switch to the router should be a non-issue if you keep the fast stuff on the gigabit switch.