Cubase sessions stops for a few seconds when seleting VEP instruments. It doesn't matter whether it's dicoupled or not. When I select non-VEP instruments, Kontakt for instance, there's no pause. When my entire session doesn't involve any VEP, such as electronic productions, no track ever stops when selected. A few seconds can be as long as 10.
I use VEP mainly for orchestral instruments since these are heavy and are in a slave computer.
My setup:
Main computer: iMac Late 2012
Processor 3.1 GHz Intel Core i7
Memory 16 GB
Slave: Think Pad Lenovo
Processor Intel Quad-Core i7
Memory 32 GB
VEP 6