I notice that even if a YouTube video is audio only with a still image for video that it still loads slower in 1080p than it does for 144p. Does anyone happen to whether or not the YouTube player loads new frames even if the video image doesn't change from frame to frame?
Even if a video is showing still image, it is still a video. It will load faster than moving images type of video since it doesn't have to refresh the frame that often, but will still have information about both audio and video stream.
That's the reason when you watch such type of video in lower resolution it loads faster in compare to a higher resolution.