There are some factors you can't do anything about SNGX. The number of switches between you and your ISP, for one. If you live a long way from your ISP then you will likely connect at a slower speed. The more switches, the more the signal degrades. It isn't as bad as noise in the line but it has a similar effect.
You can check for line noise by pressing a button on the phone to kill the dial tone, if you hear any static at all, contact the phone company and report it to them.