I've tested the RT3200 router performance with Fast Ethernet (100Mbps) devices connected to the built-in switch.
My goal was to see if there is an issue with WAN performance when Fast Ethernet devices are connected and there are LAN transfers on the network.
The results are here Severe WAN performance degradation when Fast Ethernet devices are connected to a Gigabit switch and transfer data over LAN (probably present on most OpenWrt versions) - #11 by sppmaster