Slow FTP upload over broadband

My FTP upload speed to my SiteGround FTP site is very slow - averaging 13-15 kbps over a broadband (cable) connection. Speed tests with various servers indicate upload speeds of 140-653 kbps. I have tweaked the WindowsXP registry as recommended (no impact) and the file transfer buffer sizes in SmartFTP as recommended (marginal impact). It seems to me the speed should be 5-10 times faster. What else can I do?