Not excusing apple's poor drive performance, but just to point out some of the differences.

Youtube has extensive global caching infra, both on the edge of Google's network and offnet within ISPs networks, to make sure videos are served from a location close to the user.

Also most of the videos people are gonna watch will be popular ones. I.e. they may already be filled in the cache. Compared to your icloud files which may only be accessed by you.