system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do ads attached to videos tend to load faster with better quality than the actual video I'm trying to watch?
gpt
The ads are presented to many people in your area for many different types of requests so they are kept in a local cache while your specific video likely is not.

Think about going to a car parts store and requesting a specific part. They need to go find it on the racks of many different parts. Also they give you a flyer of coupons with your order, a stack of which is kept at the front counter because they always need them for every order. The coupons are easy to get ahold of just like the advertisement is kept handy.