Assume for the sake of argument that processing is free and instantaneous, and that the cache is in-line with the normal network path (and much nearer to the end user than the content server). This is arbitrarily close to true as Cloudflare expands.

So you're actually saving these 5-200ms vs. non-cloudflare delivery. It might still be 305-505ms total page load time, or easily greater (I used GEO sat for a while, feeding cell; it was hell), but if it were 305 instead of 505 I'd be quite happy actually.

(I'm assuming a 4MB page with 100KB which actually changes between loads. A 4MB all dynamic page, frequently reloaded, which can't be cached but where a tiny delta is possible would be pretty pathological.)

With CDNs you can also get benefits from prefetch/prefill, either opportunistically (due to multiple users hitting the same thing, or through actual scheduled fill). Works for many media assets but not for dynamically generated pages, which is what Railgun is supposed to address.

That's 5-200ms from the upstream server to the downstream cache, in the event the content needs up to be fetched. This would only be affect requests for new content.

If it'd be from upstream server (or cache) to end clients, that'd be an awesome saving.