I've done a quick search and didn't find anything really related, so I'm posting this here.
I noticed a massive render slowdown when shifting a scene with Forest Pack 4.40 and a good number of VrayProxies for trees etc from Max 2014, Vray 2.50.1 over to Max2015 SP3 Vray 3.2.03.
Here are the Rendertimes:
Max2015 SP3 Vray 3.2.03 Embree enabled: ~25min/Frame; LC: ~14 min
Max2015 SP3 Vray 3.2.03 Embree disabled: ~4min/Frame; LC: ~2 min
Max 2014, Vray 2.50.1: 6.30min/frame: LC:2.45min
This is for a fly-by animation of a stylized forest scene, btw. In terms of render settings, nothing changed from Max 2014, Vray 2.50.1 to Max2015 SP3 Vray 3.2.03, except that Embree is enabled by default in Vray3, so I was fairly quickly able to pinpoint it to "Embree" being enabled or disabled. Not sure if this really has to do anything with Forest Pack, but maybe with the VrayProxies? Has anyone else noticed anything similar?