Download speeds are contingent on a zillion things, most of which have nothing to do with your computer. Several variables may have changed - the load on or speed of the server you're downloading from, the number or quality of seeds on a torrent, or the number of neighbors on the same cable line.
What are you downloading, from where, and how? What kind of speeds were you seeing before you got your new computer, and what speeds are you seeing now? Was there some sort of benchmark that you had before that you could try again now and compare?