It's geospatial data from OpenStreetMap stored in a schema optimized for PostGIS extension (produced by osm2pgsql).
BTW: Having said (to Martijn) that using Postgres is probably more efficient, than programming an in-memory database in a decent language: OpenStreetMap has a very, very large Node table which is heavily used by other tables (like ways) - and becomes rather slow in Postgres.
Do you know why it is slow? I'd give high odds that it would be a specific implementation detail in the code that is suboptimal, or maybe a design decision of PostGIS, rather than some high level architectural decision of PostgreSQL.