Thanks. I should have made this more explicit, but I care less about the WLAN performance (as long as it's ok) - I care more about the wired one.

Most OpenWRT ports don't offload the LAN/WAN routing, and doing it on a lowly ARM CPU often effectively limits this to a few hundred megabits - e.g. 150mbps on my WDR4300 (which does manage 800Mbps for the same task with the original closed firmware which does do some hardware offloading)