@Angellia What you are looking at in Task Manager is not an accurate representation of the maximum CPU usage that your VM can get. For Example, here is what my laptop currently shows for CPU under task manager:
The VM has to be under a high CPU load in order to reach the maximum documented performance. The CPU benchmarking doc lists how the benchmarks were obtained and how they can be reproduced.
Any CPU benchmarking tool should show similar results. Please let me know if you have any additional questions.