I got this bounce message from your account,
stefano.nichele@gmail.com. I'm on gmail too, but don't get a lot of
-perform messages into my spam folder.
Just in case you've got eliminatecc turned on on the mailing list
server, I'm resending it through the mail server without your email
address in it.
I concur with Merlin you're I/O bound.
Adding to his post, what RAID controller are you running, does it have
cache, does the cache have battery backup, is the cache set to write
back or write through?
Also, what do you get for this (need contrib module pgbench installed)
pgbench -i -s 100
pgbench -c 50 -n 10000
? Specifically transactions per second?