Are you using big texture images?
You're on vray, therefore rendering, for high quality renderings hi res texturws are needed, however theydemand too much on su graphics pipeline wich is built having presentations in mind.
There are ways to optimize su style for speed but there's no way to make heavy textures not slow down performance.
I dont use vray but thea. It's very easy with thea to use low res textures in su but high res on render. Probably vray has a specific workflow for that too