Well... probably it's not related to the internet connection at all (since when playing in singleplayer, the game does not use your internet connection at all, everything happens locally). At least when it comes to Steam downloads, it's most likely caused by the disk usage. I remember that I had similar problems with Steam a few years ago, while downloading multiple games (or updates) simultaneously, many programs became unusable. Upgrading to an SSD drive really did the trick.
About Rising World: The game highly depends on accessing your harddrive. When modifying the world (or when generating chunks), the game has to access your harddrive to load or store the world data. However, if accessing the harddrive becomes a bottleneck, this also slows down the game. You might not experience these issues in other games (especially not in games which have a static world).
Not sure about the streaming program though. What streaming program do you use exactly?