It's most likely caused because the host OS is being starved for CPU or RAM.
Two rules of thumb:
1) Make sure you always leave at least 4GB of RAM for OSX itself (so on an 8GB machine, no more than 4GB total across all VM's).
2) Make sure that no individual VM has more than N-1 CPU's allocated, where N is the number of physical cores in the machine. So an i5 should give 1 CPU to the VM.
#1 means that 4GB machines are really pushing things running VM's. Most of those are several years old, and either need to be upgraded, or are nearing end of life (or past it).
#2 may cause issues with Windows 10, as it wants 2 cores. If you're on a 2-core machine running Win 10, close all OSX applications prior to booting - there's going to be resource contention.
Make sure you're on the latest version of OSX (Sierra), and Fusion 8.5.3.
There are some reports that disabling 3d graphics acceleration on certain models (like the 12" macbook) can improve performance as well.
