Youtube has started to artificially slow down video load times if you use Firefox. Spoofing Chrome magically makes this problem go away.
Posted in r/youtube by u/vk6_ • 4,160 points and 371 comments

Spoofing Chrome magically makes this problem go away. The slow down is 5 secs according to the video.
Update 1: Safari faces the same treatment
Update 2: Google confirms 5 sec delays for ad-block user