Performance is always going to be something that is situation-specific.
In your case it would be helpful to know:
- What is the Guest OS you're running?
- How did you configure the VM in terms of CPU and RAM?
- Which Mac specifically are you using?
- Is there any process running away with your CPU? (check Task Manager and Activity Monitor)
Performance is typically a balancing act between what the Host needs and what the Guest requires. I usually split it down the middle for an important VM.
So, on my MacBook Pro with 16GB of RAM I'd give 2 CPU and between 4 and 8GB of RAM, but almost never more than 8.
If you'd like us to take a deeper look, you could provide the output of Help > Collect Support Information and we can examine the logs.