FYI, i have a Z800 with two Xeon X5670 running Windows 10 and the Task Manager correctly reads the processor speeds so i doubt that it is a Windows 10 issue.
Since you are having trouble with slow render times compared to what you have had before i suspect that the motherboard for some reason throttle down the CPU´s. I don´t know if there are any special settings in the BIOS when the machine is delivered as a Linux machine.
If it was my computer i would have entered the BIOS and used the "Restore Factory Settings as Default" to restore the BIOS to factory default settings. ( http://h20564.www2.hp.com/hpsc/doc/public/display?docId=c01718191 )
If that don´t do the trick, i would have tested to upgrade the BIOS: http://h20564.www2.hp.com/hpsc/swd/public/detail?sp4ts.oid=3912036&swItemId=wk_164134_1&swEnvOid=4059
The update i linked to is for Windows 7 Pro 64-bit, but discard that and run the file and follow the instructions and burn a bootable CD with the .iso file and update the BIOS upon booting the computer. It is easy! Image may be NSFW.
Clik here to view.
Report back with your results! Image may be NSFW.
Clik here to view.
/Roger
