i will admit when i first went wireless G the wireless caused the wired connections to take a back seat, even if i wasn't doing anything, browsing the internet normally and the other systems would get hammered but i would get perfect speed, the only way we fixed it was to change the router and my wireless card to a matching pair and all was fine after that,
but if its during peak times i would put my money on that, think about it, everone on your exchange is sharing the internet?