system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why do YouTube ADs buffer perfectly and instantly, but the actual video is slow and low quality?

Do they give bandwidth preference to advertisers?
gpt
Ads are typically served from different servers (often local caching servers), so they tend to load faster than the main videos (which may be served off remote servers where there is more traffic congestion and heavier server loads).