FYI:
http://blog.coelho.net/database/2013/08/14/postgresql-warmup/
Conclusion
"Warming-up a database for good cache behavior can really take a long time with pretty low performance. You must take that into account when running a benchmark. A rough estimate of the warmup time is to count how many pages must be loaded multiplied by the time to read a random page from the disk. Some ability to hint PostgreSQL about preloading index and table pages would be nice. Otherwise, resort to the above manual methods, or be patient!"