Just had a couple of issues where a Windows 2000 VM was apparently slow and consumed >80% CPU.
They had in common that they have been converted (P2V) from real hardware with multiple processors.
Now, in Hyper-V, they run with the one supported processor, but still with Multiprocessor HAL and Kernel.
After changing that to ACPI-Uniprocessor-PC, the VMs performed as expected.
To change HAL and Kernel, use Device-Manager, Computer, Update Driver, and select ACPI-Uniprocessor instead of ACPI-Multiprocessor.
(Kudos to André Roos)