Ultimately, the root cause of latency is the round trip time to figure out if the cache is valid. For large distance between client and source of truth there is not much you can do in terms of caching, diffs an other throughput conserving cleverness; it's the speed of light.
Sometimes, however, it it possible to anticipate when content will become stale: when changes to the origin don't happen arbitrarily. For instance, with live media streaming, segment chunks are usually fixed duration (say 2s or 8s for Apple HLS) and so you know your content won't / shouldn't change until...
In those cases, client (and caching proxies) can rely on expiration and do not need to revalidate which saves network traffic. But more importantly, it allows an edge server to instantly serve a cached copy to the client. The round-trip is essentially between the client device and edge server. Making that snappy might even reduce air time and save your phone battery.
Sometimes, however, it it possible to anticipate when content will become stale: when changes to the origin don't happen arbitrarily. For instance, with live media streaming, segment chunks are usually fixed duration (say 2s or 8s for Apple HLS) and so you know your content won't / shouldn't change until...
In those cases, client (and caching proxies) can rely on expiration and do not need to revalidate which saves network traffic. But more importantly, it allows an edge server to instantly serve a cached copy to the client. The round-trip is essentially between the client device and edge server. Making that snappy might even reduce air time and save your phone battery.