On Soulseek you are downloading from a single user. Your download speed is limited by their upload speed. If they can only upload at 50k/s, then you can only download from them at 50k/s.
When using torrents, you are downloading from multiple users simultaneously. So if you're downloading from five users who all have a 50k/s upload speed, then you're downloading at 250k/s.
The same thing applies when you're uploading. If your maximum upload is only 50k/s, then that's the maximum speed anyone can download from you. To ensure that you're uploading at the best speed, set the upload speed to 0. That way, Soulseek will use your connection as best it can. Setting the speed to a particular number makes Soulseek attempt to limit your connection to that number. The point of doing that is to stop Soulseek using too much of your upload speed when you're uploading in other applications. There's no point setting the number higher than your upload speed can go.
Upload speed is almost always much lower than your download speed. For instance on my connection, I can download at 80 megabits a second, but only upload at 20. That's a heck of a lot better than my previous connection where I had a 24 megabit download speed, but only 1 megabit for uploading. Ouch.
The only thing you can do to improve your upload speed is to change your ISP.
Soulseek is made for sharing music. Music files tend to be only a few magabytes in size, so it downloads (and uploads) a song really quickly in most cases. Movies, TV shows and games are hundreds of megabytes or several gigabytes. They're always going to be slow unless you have a monstrous upload speed. You're better off sharing large files over torrents.