There is an explanation either here or on Gearslutz about how DAWs tend to load CPUs. I think it has to do with how plugins are allocated as well as the total load. So the only other thing that I can think of now is trying to “re-balance” the plugin load.
So if you for example have a signal chain that consists (partially) of 12 dialog tracks feeding one dialog group, and you’ve slapped many heavy plugins on that group channel, then I’d try to break them up onto different sequential groups, or move some processing back onto the original audio tracks. It’s a bit clunky and not intuitive, but I seem to recall that processes get allocated to cores partially according to where the plugins live. So that might be worth investigating.
(The above is what I’m thinking because you mentioned one core being close to 100%, which I guess could be a bottleneck.)
And actually on that note I’m wondering if there are any Nuendo settings that could change things. There’s the toggle of “multi-processing” (which I’m sure you’ve selected) and “VST guard” (which I know little about) etc under “device setup”… Have you messed around with those as well?