How to deal with seldom used big database and postgresql?

I loaded in PostgreSQL (9.3) OpenStreetMap data for whole europe (400gb). Then I installed a geocoding api (nominatim) that queries the database with gis queries. My problem is : > This database is queried a few times a day, but because postgres loads > the data in its buffer on demand, my first query on a particular gps > point is always slow, and I do only one query per GPS point, so it's > always slow (like 60sec slow against 100ms when the data is buffered) . What could I do here ? - I'm using a A6 Azure instance (4 Cores, 28 GB memory, HDD). - Server is Ubuntu 14.04.4 LTS - Swapping is enabled - There is index on the geometry columns queried. - The shared buffer is 10GB - work mem is 256MB - maintenance work mem is 4GB