The first example is more likely congestion than throttling, but that's only really a difference in the manner that your ISP is being cheap (and not having enough bandwidth upstream).

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.