It is a calculation based on your own download speed available, the upload speed of the various hosts you connect to and the quality of the connection between (IE crappy connection means more bits have to be resent)

Rough download speed guide
256dsl - 25kb/s
512dsl - 54kb/s
1500dsl - 160kb/s

Remember though most DSL usr upload speed is much less than their download IE
256dsl - 6kb/s
512dsl - 12kb/s
1500dsl - 54kb/s
is not uncommon, and that does not allow for crappy connections.
So for you to get a high download, say 150kb/s, you need to connect to 8 people with better than 20kb/s upload speeds, not servicing any other uploads.

So if you want those sort of speeds, probably best to set you search filters to T1 or higher, and give serious consideration to setting quality a bit higher as well.

Then if what you are searching for is not to popular, you might stand a reasonable chance of high download speeds if your search finds 15 to 20 hosts available. (IE hopefully about half of them not servicing other uploads)