It would be your operator most probably...

Mobile broadband (3G and 3.5G) isn't the fastest or most reliable at the moment because the bandwidth isn't there, and with lots of iPhones, 3G dongles and other smart phones all connected at the same time during the day its going to put a massive strain on it.

I haven't got anywhere near the 7.2mbps that 3.5G is supposed to give. I'm on O2, and on average I get between 1 and 3 on a good day.