system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it that ads on YouTube load and play almost immediately and in HD quality, but the actual videos will buffer, loading very slow and pixelated? (regardless of length sometimes)

Follow up questions:
Why do full-length movies on Netflix load faster?
Why haven't the YouTube developers upgraded the performance by now?!?!
gpt
It has nothing to do with the performance of YouTube - their servers run well. The ISPs that are between you and the YouTube servers are using something called QoS (Quality of Service) to prioritise the traffic that goes across their networks. They prioritise YouTube's traffic very low, and what you experience is long load times and buffering and stallouts.

This can be tested by using a VPN to connect to a VPN endpoint outside of the ISP you're using, and streaming a YouTube video — it will stream significantly faster. This is because your ISP cannot inspect the VPN traffic (it is encrypted) and will only throttle it statically instead of dynamically. Or the ISP will not throttle the VPN traffic at all.