Case 1: It could 3dr party security software, for example Anti virus software. Disable it for a test.

Case 2: If it is Windows 2003 or XP, make sure you install the Integration Services or re-install it.

Case 3: Make sure you are using the "Network Adapter" and not the "Legacy Network Adapter". The legacy adapter has a lot of emulation which causes lots of CPU overhead.

Case 4: If you use dynamic memory, disable it for a test.

Case 5: If you setup VHD dynamically expand, try to use fixed size.

Case 6: Remove the CDROM drive from the guest if you don’t need it.

Case 7: If this is converted VM from a live physical machine, make sure there is not auto start programs running. Check the performance.

Case 8: Add more RAM and CUP.

Case 9: It could be Virtual Machine Queues issue. Please check this link: VM on Hyper-V is slow - Resolution with screenshots

Case 10: Disable NIC offload on both host and VM.