Twitch streaming is purely outbound. Most cable and DSL plans cripple uploads, a 300 Mbps cable plan often only gives 10 Mbps up. Fiber is the gold standard for streamers because uploads match downloads.
Always reserve 30-50% upload headroom. If you stream at 6 Mbps, you need at least 10 Mbps of stable upload, anything less and OBS will start dropping frames during action scenes when the bitrate spikes.
Twitch caps non-Partners at 6 Mbps. Pushing higher won't help, Twitch will throttle the stream. Use x264 medium preset on capable CPUs or NVENC on RTX cards for best perceived quality at fixed bitrate.