For a very small instance with only a couple of concurrent users a CDN might not make much difference. But if you take a look at your web server logs you’ll quickly notice that every post / like / vote triggers a storm of requests from other instances to yours, looking up lots of different things. It’s easy to imagine how quickly this would overwhelm an instance once it gets even a little busy.
One of the first web performance tools people reach for is to use a CDN, like Cloudflare. But how much difference will it make? In this video I show you my web server logs before and after and compare them.
Federation and decentralization is not what the users of bsky want. It's (for some) a nice to have thing but way down on the list of what they value.