Just as fast (and that means pretty fast). Comparing to MongoDB might not make sense, if you're running it with no fsyncs (I don't know much about MongoDB, so maybe it is possible to make it fsync data).

There was a good talk about using Postgres as a key-value store at PGcon 2010, see http://www.pgcon.org/2010/schedule/events/219.en.html and http://www.scribd.com/gavinmroy/d/31669670-PostgreSQL-and-No... (especially the classic photo of DBAs running with scissors)