Hi,
Care to share your Task Manager Windows 10? Here's mine: Mac Pro 5.1 (2012) OS X 10.10.5 [8192MB/24GB & 6/12 cores assigned to WMware Fusio 8.0]
1. Do you get the black screen - after the Windows logo - in the VM or also booting the bootcamp partition native?
2. Do you have special applications running when you experience the lags?
3. Did you already update your Windows 10 to the latest 10586.14 release?
Cheers