I worked for an ISP (in Russia), and in the early 2010s, we had quite a lot of complaints about YT videos buffering. We've checked our network very carefully and found no signs of any trouble. And we've confirmed issues and were able to reproduce them even when the network was least busy. We've tried to re-route traffic via different uplinks, even messed with the DNS records a little bit (I admit this grave sin) - sometimes with some temporary improvement effect, but mostly to no avail. And of course, we've talked a lot to our peers. This was quite a major headache, and we were out of ideas what else we could do (we weren't anywhere large enough to get a Google Global Cache server). Then Google did something (IIRC, mid-2012), and we've stopped getting the complaints.
With VPN you're probably hitting different YouTube servers. I guess we should've tried to route via some VPNs too, not just physical peerings we had. Never occurred to us at that time.
I left that company in 2015. Don't know how things are now.
My point is, even without a malicious intent sometimes things look the way you describe.
Using a VPN in this case is probably causing your traffic to take a different route than everyone else's youtube traffic, thus avoiding the congestion bottleneck.
Netflix may have a local cache nearer to you, so you don't hit the congestion (and the amount of content in a Netflix cache is probably a lot less than Youtube).
Example 2 could be a packet-lossy network which causes your TCP connection to ramp back. If it's constant packet loss, your TCP connection will probably get slower and slower: TCP uses packet loss as a signal it's going too fast, but if that's not the cause of the loss, your connection will just keep getting slower.
Doesn't that depend on the peering agreements? How do you know it isn't the case that problem at the links isn't due to the video service paying for insufficient bandwidth?
Edit: downvoted for asking a question. Thanks reddit.
This is used for congestion control: Imagine you have a computer with a 1gbps LAN hooked up to a 100mbps network. You can send packets at 1gpbs over the LAN, but 90% of them would be dropped at the speed change point. The TCP stack will notice it is losing packets, and will throttle itself accordingly.
TCP is actually more complicated than just this, but this is one of the more important parts.
Netflix small fixed library is significantly better for cache hits.
Google and Netflix peering boxes mean that ISPs with peering boxes at the same peering point can download as much as they want, within reason, for free from that peering box.
You might be confusing CDNs with peering boxes. As might I.
I'm sure it's a technical issue or something, but that is my experience with Centurylink and Youtube.
Now I use Verizon and only have issues with youtube when it's "congested."
Recently it seems it gets damaged every four months or so. [1]
[1] https://en.wikipedia.org/wiki/SEA-ME-WE_3#Service_disruption...
I have two examples:
1. YouTube vs Netflix. When I would try to watch YouTube in the evenings it would not buffer fast enough for continuous playback. While Netflix would. I assumed that my ISP was prioritising Netflix traffic. When I turn on my VPN YouTube plays much smoother in full quality.
2. Steam downloads. When I would download a game off the automatic download server is would start at full speed (Blistering 100mbps on NBN...) and after a few seconds drop down to single digit kbps. When I switched the download server to the Singapore one I would get constant full speed downloads. Could either be throttling that specific server, or there is something wrong with that server.
I switched ISP and now both of those issues are gone.
From my understanding that points pretty strongly towards throttling. Though admittedly my knowledge may be lacking.
Note: This is for a wired connection. But I thought it was relevant anyway.