For example, it is very easy to write badly performing code using ORMs. And yet, ORM is often chosen for good reasons initially to give development speed (e.g. Django forms) The problem with quickly prototyped ORM based apps is that initially the performance is good enough but when the data grows the amount of queries goes through the roof. It is not the amount of data per se, but number of queries. Fixing these performance problems afterwards for small customer projects is often too expensive for the customer, but if there were a plug'n'play in-memory SQL cache/replica with disk-based writes, it would easily handle the problem for many sites.
Configuring PostgreSQL to do something like reading data from in-memory replica is likely possible, but I see that there would be value in plug'n'play configuration script/solution.