I haven't had any issues with slow buffering yet, but I have experienced something different: Sometimes the video playback suddenly stops altogether in the middle and throws me out of the video completely. I suspect that an ad is supposed to be played in this particular part of the video. I have to re-start playing the video from the part where it threw me out. Sometimes this happens several times in the same video.

I use the latest Firefox in combination with uBlock Origin (also always up-to-date).

Edit: typos