dd if=/dev/sda of=/dev/null creates ever increasing load every second.
After minutes the load has moved up to 4.79
I've tried with two different discs in my system.

I wonder if it's a delay or problem with the SATA bus because one drive I have connected has recently failed.
I notice when drives fail that you get some IO/blocking issue when they don't respond properly.

Yes I believe it was, because here's the same disc after removing the dead SATA drive from the SATA bus:

 03:49:32 up 11 min,  2 users,  load average: 1.18, 1.00, 0.52
 04:00:27 up 22 min,  2 users,  load average: 1.18, 1.15, 0.84
 13:22:19 up 44 min,  2 users,  load average: 1.01, 1.12, 1.07

It's too bad Linux doesn't have a way I know of to tell the SATA bus to stop probing a dead SATA drive/port.
This feels almost like the blocking IO that you can get between IDE Slave/Master when one drive malfunctions.