system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 how adverts before YouTube videos never have buffering/loading problems, even on a slow connection.

Just curious, as I'm on a slow connection right now while I'm house-sitting and even short, low quality videos take abysmally long to load while the pre-video spots load instantly.
gpt
It might have something to do with the fact that the average youtube ad is viewed *very* often.  Generally web servers (or really most servers) make so content accessed more often is easier to get to.  So a really popular video could load much quicker than some obscure video with 50 views, latest back in december.  This may also apply to ads.

Sorry if this isn't a good answer...it's more of a guess.