I noticed these issues since about 12/2012 ... I got Bright House (TimeWarner) + smart phone on AT&T's network and videos are absolutely slower up until now. Some videos constantly buffer and play a second or two and then go back to buffering ...even 240p quality.
Gotta test some of these "tweaks" to see if anything helps.
It started about two years earlier in Canada (though few had high enough bandwidth to stream 1080P in Canada) and has finally spread to the rest of the world years later in the future. 100 percent certain the isp's in Canada throttled youtube but finally it appears youtube is throttled at the source now.