The TiFlash replication is sub-second and happens automatically. We are going to be publishing a more extensive paper on TiFlash, but the point is that the operational burden is equivalent to a read replica but you get the column-based storage speed of a data warehouse.

I agree the data size doesn't seem extraordinarily big from some of the description. Later in the article it states a size of 2.4 TB.

It is also true that in-memory databases should be faster. TiDB is designed for data that is too large to keep in memory.

Note that having 5 TiKV nodes isn't going to make this single count query any faster than having 1 TiKV node. TiDB is designed for high availability, so a default TiKV setup has 3 nodes each with a copy of the data. TiKV can be scaled out horizontally to provide more compute power to handle a larger workload but it is not likely to make a single query on an idle machine any faster.