IMHO if my main concern would be performance, i definitively would go with a commercial product, even with Linux, i would buy a Linksys WRT54G, and switch firmware to some OpenWRT or DD-WRT.
Because… using ZeroShell even in the best available machine on the market is still a slow solution, remember that a router like the Linksys already have dedicated hardware for routing, and internally they know that the hardware will not change, that’s a very big advantage for “low level” programmers.
it’s like putting a Ferrarri engine(the best software) on a car with no aerodynamics(Hardware that was not tunned for that specific job). The thing will not perform as good as a Ferrari(as the Hardware that was prepared for routing).
Best regards. Joel Rocha.