String concatenation/substitution is more expensive than DOM parsing?
> Only one user will see the 810ms latency.
If your whole content is cacheable, then a CDN will be far more useful and have way less than 20ms response time. 20ms is not fast. Selectively invalidating CDN content as new data is available is much more performant.
> Have you ever worked with servers/caching
Yes.
> know how they work
Yes, still learning though.