The truthful answer is that ISP's are throttling Youtube through DNS filtering and scrambling so they can have more bandwidth for advertisements and information gathering about your online browsing habits
9/10 ISP's are doing this worldwide, a good suggestion is to change to openDNS servers ,either 8.8.8.8 / 8.4.4.4 or other
(https

/use.opendns.com/)
using a VPN server or even visiting Youtube through TOR can increase the buffering up to 400%)
Moving the browser cache to a faster hdd/ssd or even a RAMDisk helps too
when doing the cache option a little math (a 1080p video of 60 minutes would be app. 8-12 GB of RAW vaudio/video data cached so have at least 2x that on the RAM/HDD/SSD for the caching)