AFAIK that is youtube delaying the video stream on purpose because you were supposed to watch ads in that time. If you wait 5-10 seconds it will start playing. For me it happens only on the first video, and autoplay is still instant afterwards. I am gladly waiting out the loading screen than watch ads. Knowing that I probably make google lose money on every view makes the wait even more bearable (I am never logged in, and anyways I always run youtube in private mode, so there's almost no profiling happening)