CloudFront is a pull CDN, meaning that it fetches a file when someone requests it so that next time it's physically closer to them. It is not a magic "make it faster" panacea.
If a file is expected to be requested by a lot of people in a lot of different places, like on a popular website, it'll make it faster. If it's being uploaded by one person and downloaded by one other person, like in a chat app, it'll probably make it slower.
A push CDN might make everything faster all the time, but those are even more expensive.
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