Processor always at max speed

I noticed the same issue when Hyper-V was enabled.

Hyper-V works by running a hypervisor layer between the bare metal machine and all operating systems running on it - including the host. It is this hypervisor layer that interacts directly with the system including the CPU, and so the host OS can only see the hardware that the hypervisor reports to it:

In this situation, the hypervisor reports to the OS the full speed of the processor and so it appears that dynamic multipliers (which is how the system reduces the CPU speed under low load) are not working however they are actually still active and being controlled by the hypervisor itself.

As such, there is nothing you need to do as the system is actually behaving as you want at the hardware level.

Task Manager will show the maximum capable speed, not necessarily the current one.

Try seeing what CPU-Z reports.