Probably not much you can do about it. You might check at your cable isp for any problems in the area. There are a few tweaks you can do to your machine that can improve your speed but it won't be anything drastic. The main slowdown with a cable connection is having too many people within your neighborhood signed on. Cable bandwidth is shared by everyone in your node. In my node, when the Kiddies get home and early in the evening when a lot of people seem to be online there is a perceptable slowdown. This is another thing you can check with your cable company about. Complain loud and hard to them that your connection is not getting the speeds you are paying for and you just might get some help.
Here's a link to some helpful tips on speed and below that a couple of links for testing.