Wireless d/l speed only half that of wired.

Although I'm very pleased with the boost in Internet speeds we got this week, I'm very puzzled as to why the wireless d/l rate in all three of our computers which are wirelessly connected.
I posted this to the MacRumor forums and got a lot of replies. As yet, however, the problem remains unresolved.
Nevertheless, There are very many articles, posts and threads on this topic (including this, this and this) but all without an obvious solution for my situation.