Speed of streaming service
You should aim for HD quality at 11 Mbps for smooth streaming on Twitch.
Twitch limits the bitrate to 6 Mbps, meaning the quality you perceive as suitable fits within that setting.
Select a bitrate between 12 and 18 for optimal performance.
6 Mbps is the maximum Twitch supports. Whether 720p at 60 FPS works well or not relies on various factors. Refer: