> Buffers: shared hit=544 read=6760 dirtied=4034
> I/O Timings: read=69709.611
You has very slow (or busy) disks, not postgresql issue. Reading 6760 * 8KB in 70 seconds is very bad result.
For better performance you need better disks, at least raid10 (not raid5). Much more memory in shared_buffers can help
withread performance and so reduce disk utilization, but write operations still will be slow.
Sergei