Exact performance numbers are specific to your deployment, how the application works, and the state of your infrastructure. Get real user monitoring going, and get numbers about the experience before and after changes.
Web sites, even simple ones these days, have many different objects to fetch. Framework code, common content, images, and yes dynamic content. All but the dynamic per user stuff can be cached to some extent, if your origin server provide metadata like headers. CDNs make use of that to provide caches, and CDNs with global networks try to place them closer in latency to the user.
A good CDN would be very well peered on the internet, such that traffic would have a fairly direct path to most places. So taking a short detour to on ramp the CDN "highway" is likely worth it to gain the various benefits the service provides. Making numbers up from nothing, maybe the cache point of presence is 30 ms away, and the origin is halfway around the world at 170 ms. Add some performance optimization of the web site, even with some cache misses I find it plausible you can get something to appear to the user in under a quarter second.
Based on your question, a user type to watch is crawlers, and perhaps first impressions in general. Monitor the performance of users that aren't logged in or otherwise have a custom experience. Most or all of this probably can be cached, so if other users have warmed the cache already it could be all hits.
