Earlier quoted context omitted.
Your ISP? Youtube takes 10-20% of typical internet traffic (more than bittorrent) and competes with telecom TV offerings.
My ISP doesn't throttle, which is entirely why I picked them. I have access to a connection that downloads 20 megabytes per second from S3, and YouTube still buffers on that connection. Note that Vimeo doesn't buffer, and Vimeo has higher video quality usually. Also Netflix doesn't buffer. YouTube has been getting worse with time rather than better. Tons of ads and slow video. The issue is definitely on the YouTube e…
That doesn't mean that they have infinite pipes to Google's edge caches, though (you're not exactly the only one pulling data through those pipes :-). Or that they're even pulling their data through the right pipes. I suggest talking to your ISP. If enough users complain to them, and the problem isn't in their network, they'll complain to Google.