Just so you know, 20 MB is not 20 MB broadband, it's 20 Mb, there difference being that there 8 b in a B (8 bits in a bytes).
So on a 20 Mb broadband connection your maximum possible speed is 2.5 MB/s, so you're very very lucky if you're actually achieving that speed. I don't know which service your with, but if it's on ADSL2+ there are a number of factors which can slow it down, distance from the local exchange and the type of weather are 2 big ones. But even if you were sitting next door to your exchange and it was a clear day, I still wouldn't expect 2.5 MB/s out of the connection just connected to a single source (downloading over http or ftp). Your more likely to max out on a private torrent site, or if you run multiple downloads at once.