Does sqlite cache pages in memory?

If not, how can it be faster?

Is it the IPC overhead of Postgres?

Yes it caches pages in memory. The cache size is configurable via a PRAGMA.

Postgres / MSSQL / all RDBMS is slow because of network I/O.

>Postgres / MSSQL / all RDBMS is slow because of network I/O.

I assume in situations where you're choosing between Postgres and sqlite, everything is running on a single machine anyways.