Throughtput strangeness

I have a RB532 with 2 Atheros cards, both on AP mode, linking to a single client each. It is just a hop with 2 Point-to-point links because of a hill in the way.

Now if I do a bandwidth test (from RB532) to the one link, I get 10Mbps, to the other, I get similar speed, but when I test from one client to the other (through the RB532), I get only about 2mbps. The 2 frequencies are removed 5 channels apart. No firewall rules, mangle or anything else, pure routing.

Is the RB532 just not powerfull enough or is there something else I should be looking at?

Thanks

Ekkas