Hi
The problem appears to be transmission over the SATA bus. Basically the data leaving the drive is being received with lots of errors, so your computer is having to re-request the data many times and this is causing your slow downs. This could be caused by various things:
1) Bad SATA cable or bad connection. Try a different SATA cable and SATA port.
2) Cable is running past a component or other wires that are causing interference, so try making sure the SATA cable is routed away from other cables.
3) A fault in the SATA drive or the motherboard.
You can sort out 1 and 2, but if that doesn't help looks like something is faulty.
Regards
Phil