PostgreSQL synchronized commit performance
Don't do this with only one slave.  If you do you are making your application more subject to failure, not less.  Basically if you do this, and you lose your slave, your master will hang.  You say your backup box is crappy, and I assume that means reliability-wise too, so this is more likely than losing your master.  This is probably not what you want.  Do not use synchronized commit option with replication and only one slave.

Synchronized commits in replication means that the master waits for the slave to acknowledge that the WAL was applied before noting that the commit happened and passing that back to the client.  If you don't get the configured quorum, you wait for it, possibly for.... a..... long..... time......

Do it async.  Use fsync set to on.  You should be golden.