Slow SQLite performance

[themagician]

1) The database file is 455MB in size with 6 large tables with only a few columns in each, but millions of rows
2) I am using prepared statements
3) I am using transactions
4) Database was default created with:

QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE");
db.setDatabaseName("filmquery.db3");
db.open();

QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE");
db.setDatabaseName("filmquery.db3");
db.open();

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5) The tables have proper indexes and all the queries run in 0-2ms when run from e.g. SQLiteSpy

My app needs to do a lot of DB queries and when I first run the database querying function it takes a very long time to execute the queries and you can hear the hard drive reading a lot, but when I run it again then it's very fast, like it caches the whole database. Could I e.g. load the database in memory when I load the app? What optimization tricks are there to make it faster?

Thanks.

edit: Vacuuming seemed to work.