Well, the conversion process into the JSON representation is going to take CPU, and then you have to store the result, in essence doubling your cache footprint.

Doing it on demand still utilizes their cached version, so it saves a trip to the origin, but doesn’t require doubling the cache size. They can still cache the results if the same site is scraped multiple times, but this saves having to cache things that are never going to be requested.

Cache footprint management is a huge factor in the cost and performance for a CDN, you want to get the most out of your storage and you want to serve as many pages from cache as possible.

I know in my experience working for a CDN, we were doing all sorts of things to try to maximize the hit rate for our cache.. in fact, one of the easiest and most effective techniques for increasing cache hit rate is to do the OPPOSITE of what you are suggesting; instead of pre-caching content, you do ‘second hit caching’, where you only store a copy in the cache if a piece of content is requested a second time. The idea is that a lot of content is requested only once by one user, and then never again, so it is a waste to store it in the cache. If you wait until it is requested a second time before you cache it, you avoid those single use pages going into your cache, and don’t hurt overall performance that much, because the content that is most useful to cache is requested a lot, and you only have to make one extra origin request.