Choose a speed limit depending what you need to do with your connection:
• If you just need to browse the web, limiting Steam to 80-90% of your internet max speed should be okay.
• If you need to watch HD videos online (YouTube...) or live streams (Twitch...), you may need to limit Steam to 50-70% of your internet max speed.
• If you need more, just choose a lower speed in the list!