I have a 1024/512. BitComet 0.70 works like a rock for me. I have set the settings as advised in The Settings Guide on this board. Some minor changes applied.
Use v 0.70 after uninstalling 0.77. You’re getting your units confused, as your line is measured in kilobits, while BitComet uses kilobytes. Downloads from a file server via ftp or http are nothing like bittorrent, and the speeds are not comparable or even indicative. Your net slowdown is due to failure to configure your client correctly, and to cap your global maximum upload speed. Read and follow the settings guide to configure it properly and to set your maximum upload speed correctly.