there are small, low energy chips in the router. Creating a fast network is not that hard. Encrypting data is however a cpu hungry task, so yeh, it always slows down things, but it's better than pumping private data in the air so other people might have a look at that.
I have a intel AGN card, but with WPA2 enabled I can get up to 6-9mb/s quite good comprared to 54mbit networks.

my advice... try a different "Regulatory Mode" in the advanced wireless settings