Initial queries of day slow

Hi all.

I'm a bit stumped. At present I'm finding that queries to my database, that normally execute promptly, are taking a long time when they are executed first thing in the morning (after the database has been inactive for several hours). After the first execution, everything is back to normal. 

A while back I turned autovacuum off and now instead I run a daily cron at 3am that executes a script which does a VACUUM ANALYZE on each table. 

These are my details:

Would love peoples opinions on what the issue could be.

Thanks