tl;dr : Popular videos are cached at a server geographically close to you, so you connect it at near quoted speeds. Less popular videos may be cached further from you, so you connect to that video at a lower speed than quoted by your ISP. With a 400Mbit connection, even with this speed drop you have enough bandwidth to stream videos at high quality, but with 5Mbit the connection to the server hosting the less popular video may not be fast enough to handle higher streaming quality.

 Long version :

 Could have more to do more with your ISP change than Youtube being nefarious.

 Youtube, like most of the web, uses a Content Delivery Network (CDN). To minimize the distance between the user and the data the user wants, a CDN stores a cached version of the website in multiple geographical locations (a.k.a., points of presence, or PoPs). Each PoP contains a number of caching servers responsible for content delivery to visitors within its proximity.

When a user requests for data from a website, the nearest PoP will handle the request. If the nearest PoP hasn’t cached the data the user is requesting, you move up PoPs on the chain until you reach a server that has the data the user wants.

Popular videos like LTT are almost assured to be cached at the closest PoP, but the less popular videos may only be stored at one of Youtube’s “real” data centers.

 Your actual connection speed to the nearest PoP will be much faster than your connection speed to a server further away. That means you can stream a more popular video cached at the nearest PoP at a much higher quality than a less popular video cached at a PoP somewhere further away.

 With a 400Mbit connection, you may not notice this drop in connection speed affecting video streaming but at 5Mbit you may notice an effect.

 If you want to test this out, go to [speedtest.net](https://speedtest.net) and check your speed when connecting to the optimal server on their list and then connecting to a server in San Francisco. I have a 20Mbit connection, and the nearest server connects at 19Mbit but the AT&T server in SF only connects at 12Mbit.