Ok, so in VE Pro 6 on my slave PC I started creating empty instances in blocks of ten, then creating instances of the VE Pro 6 plug-in in Logic and connecting them. Starting app. around 35 is where I start to notice the latency. At 43, it is just a little less than with my populated instances.
I reproduced this both locally and connected to a PC with 44 empty connected instances. Locally, there is about half a second of lag before the transport starts playing, which I'd say is acceptable considering the rather large amount of connected instances. Connected to a PC, there is indeed about 1.5-2 seconds of lag, which has to be due to the extra network round-trip per instance.
The strange thing is that this part of VE Pro 6 has not changed. If anything it should be faster than before. Are you absolutely certain this lag didn't exist in VE Pro 5?
I am not answering for Ash, but I can say that there is no lag whatsover (well, yes, maybe - but it's neglectable - maybe 0,2 seconds) in VEP5 for me, no matter if I only have local instances or both local and slave. Typically my template will be around 16 local instances + 6 instances on one PC slave + 8 instances on another. No lag in VEP5.
I am on a Mac Pro, latest El Capitan, latest Logic etc.