> How long S3 takes to fulfil a request does not affect bandwidth.

Correct. Did I insinuate that? I apologize if I did. They are two distinct issues, both of which a CDN prevents.

1. S3 outbound bandwidth is expensive. Use it only as an object store of last resort. Your CDN bandwidth is orders of magnitude cheaper (don't believe me, go compare the pricing).

2. S3 response times can vary wildly at times. Use a CDN to avoid this.

And of course feel free to use a cache key instead of invalidating via an API if ~15 minutes it too long to wait for fresh content to appear at edges.

PS Don't apologize for laziness. When directed appropriately, its a most productive force.