So you're getting a 7.1Mbps service delivered to your door and getting as far as your router. That's pretty good really. I wouldn't say you're losing much speed. There will of course be latency issues on how you're connecting to your router but that's to be expected - especially if you are connecting via 802.11g Wi-Fi.
So....its quite acceptable to be getting 7.1 Mbps to your router, but only be recording speeds (from speedtester sites) of about 3.9 Mbps? its not far off half the speed coming in and to me...thats a fair drop
if it makes any difference, im using the white BT Home Hub. I also have the black one, lying unopened upstair.