YouTube – Shortcut to Focus Comment Input

Your Internet speed is too low, as you said animated circle, that's indicating you that the video is currently buffering, in another words, it is loading, so whenever you open a video page on YouTube, play it, than immediately pause it, let the video buffer, you'll see a grey line filling, once it's filled, you can play your video without any interruptions.

It's the forward-slash key (/).

This also works for searching in Google Maps.

Source for this and other YouTube keyboard shortcuts:
https://support.google.com/youtube/answer/7631406?hl=en