Re: What limits Postgres performance when the whole database lives in cache?

It is not about durability only.

Postgres holds data in format equal or similar to saved data on persistent storage. There are repeated serialization and deserialization. Some structures are designed to be simply saved (like Btree), but the performance is second target.

I believe so new memory databases can be 10-100x faster - depends on use case, because they hold data primary in memory and uses different data structures. The performance of these databases is great, when all data are well placed in memory all time. But the performance is pretty bad, when this rule is not true. There is another issue - when you increase speed of database write operations, probably you will hit a file system limits, spin lock issues - so it is one reason, why big system are based on distributed systems more and more.