This condition occurs when the System Preferences Energy Saver Put the hard disk(s) to sleep when possible checkbox is checked (on), or the Disk Sleep Timer is enabled; the Tempo-X SATA card using firmware version 1.2 does not support disk sleep.
To correct this issue, update the Tempo-X card's firmware to version 2.0 or later. Download and run the installer to add disk sleep support.
Another manufacturer's previously-installed SATA driver may be accessing a Sonnet SATA card in error causing a drive access slowdown. For example, we have seen Fusion D400QR5 performance drop from 200MB/s to under 50MB/s in such circumstances. The solution is to remove the other manufacturer's SATA driver.