How do I remember a speed for one channel?

Speed is a Pro feature.

1. Open a video from that channel.
2. Tap the speed button, and slide to the speed you want.
3. Turn on Remember for this channel under the slider.

Every video from that channel then starts at that speed. The panel names the channel the speed belongs to.

To go back, turn the switch off. The channel returns to normal speed. In Settings, Speed per channel only counts the channels with a speed. You set each one from the player.

See Player controls.