Why does resetting a wireless router often result in improved wireless connectivity?

I have a standard N wireless router (Netgear). It works fine, but occasionally it really slows down. I can still connect, but the connectivity is terrible.

After I do a complete reset of the router (unplugging it, waiting and plugging it back in) it works fine, until the next time it doesn’t.

I’m just wondering what’s going through the router that slows everything down. I could understand if it were an IP conflict, such as I had another device that it was confusing it, but I don’t. I can still connect—it’s just slow.