The downside of this approach is that it only works with popular nodes and you'd be back to the old, centralised internet architecture for all real use cases.

I don't think you can accurately gauge what is and isn't popular in a P2P network like IPFS. You never have a view of the entire network, after all.

There's also the problem of running such a system. Who pays for the system's upkeep and do we trust them? If we'd use Cloudflare's excellent features, who says Cloudflare won't intentionally uncache a post criticising their centralisation of the internet, forcing the views they disagree with to the slow net while the views they agree with get served blazingly fast.

I don't think such a system would work well if we intend to keep the decentralised nature of IPFS alive. Explicit caching leads to centralization, that's the exact reason caching works.

Instead, the entire network needs a performance boost. I don't know where the performance challenges in IPFS lie, but I'm sure there's ways to improve the system. Getting more people to run and use IPFS would obviously work, but then you'd still only be caching only popular content.

Edit: actually, I don't really want to see caching happen through popularity of the service either, because as it stands IPFS essentially shares your entire browsing history with the world by either requesting documents in plain text or even caching the documents you've just read. I wonder if that IPFS-through-Tor section on their website ever got filled in, because the last few times I've checked that was just a placeholder in their documentation.