Abnormally slow CPU speed in Windows 10 Task Manager, with virtualization disabled

Question

In Windows 10 Pro Task Manager, CPU speed shows a constant and unchanging value of 1.18GHz, while nominal CPU speed is 2.50 GHz. Dual Intel Xeon E5-2680 v3, on a Intel S2600CW2S server board.

Per this question, I turned off CPU virtualization in the BIOS. After rebooting, my CPU speed increased back to its typical, fluctuating normal range (~3.0 GHz, not overclocked). System performance on timed tasks indeed much faster than when CPU speed showed 1.18 GHz.

Unfortunately, with the next boot cycle, the CPU speed decreased back to 1.18 GHz.

During the single work day that the CPU speed was back to normal, I installed Bitdefender 2016 and ran a system scan. I also installed Visual Studio 2015. I hesitate to speculatively uninstall: how could either of these applications conceivably degrade steady-state CPU speed, in the same way as a processor BIOS setting?

In Control Panel / Power Options, I verified that I have the High Performance plan selected, with minimum Processor state set at 100%.

I went back to the system BIOS, and cycled virtualization on and off again (with intervening reboot). No change, still stuck at the lower speed. Google search shows that 1.18GHz is a common problem. There seem to be many ad hoc folk cures out there, but none worth repeating here.

Any assistance or suggestions appreciated, please.