Slow uploaders don't necessarily waste upload slots. Check out the options in your tools->options->uploads window. LimeWire will initially give out "start upload slots" uploads. Then, as long as there's enough bandwidth, it will continue to give out uploads until it reaches "max upload slots". Be careful not to set "start upload slots" too small, our LimeWire won't be able to tell whether it's you or the downloader that's run out of bandwidth.