From Google Cloud's document
Cloud Storage essentially works as a content delivery network. This does not require any special configuration because by default any publicly readable object is cached in the global Cloud Storage network.
My current web application is already using CDN. It contains a fair amount of video content. The video content is publicly accessible. Will using a combination of Google Cloud Storage and Cloud CDN further improves latency?
The answer is it depends. Caching is a complicated item. How popular is your file? How large is your file? Are your clients in one region or global?
All of those factors and more will affect whether your files are cached or delayed due to cache refills.
In general, data that is delivered via Google's internal backbone will transfer quicker as fewer network hops are traversed and have fewer public points of failure. By using a CDN, data is more likely to stay on Google's backbone because the entry point is the nearest Google POP (Point of Presence).
