Working with Alpha 6
Preview for New Material needs very very long time, although in quality 2.
A new materieal preview needs 10 seconds. I would say this is a disaster
have you an another render engine installed ? Make a fresh new install of c4d and test 
Yes, I also have Vray 3.4. It is the latest C4D R18 Version, which is new installed.
The Problem is with C4D R17 and 18. My sytem is running full speed for Corona.
check: Slow Material Editor - C4D
What do you mean with slow Material Editor? Is there something, I can change, except the quality
of the previews?