Same here. Two days ago I know most Google resources were running quite sluggishly for me, and YouTube was a mess that day. The standard tricks of reloading the page or toggling video quality between two options didn't help. It's been fine since. Much of the YouTube slow deal is on Google's end from what most have noticed since simply reloading the page can often fix speeds. Other times it's faster on more popular videos where lightly viewed videos take longer to load, due to the wa Google caches videos. It does depend a ton on the routing though, and the more customers with faster connections a provider has in a single region, the more likely it is for YouTube for see issues for one or more providers in that area if they hit the same datacenter or cache servers. Customer count is also a huge factor, too. The more customers, the more likely for issues.

Chances are much of the issue stems from the fact that most providers send their traffic to a single regional/nearby POP, most customers use a single set of regional DNS servers, and Google has a datacenter in or near the regional POP that handles traffic from that POP for lower latency. Now, you're dealing with a few things from here. If all of the providers going to that POP have very fast connections that overwhelm Google, then the problem starts to bite Google. If it's only one provider that has very fast connections and everyone else uses Cell Phone Internet, you'll find the provider with higher speeds will likely see problems. Also, each provider routes differently and also hits a different set of servers/cache farms, so if a provider has for example, high speed connections and they're hitting a specific set of cache farms only, then those servers get punished and it seems like a provider issue.

YouTube has a lot of work themselves as far as what they need to do in order to improve the speed of YouTube but it's a multi-faced problem. I can't describe it all in a forum post.