Can be bad programmed plugins as well… look at Slate FG-X Mastering processor… a real CPU hog… Asio Meter jumps 30% when loading that plugin - windows tells somewhat about 2 % … I am sure this can be done better.
Just create a project with only Nuendo plugins… ad plugins till you have reached a reasonable Asio load… assuming you are not on 32 samples buffer or something (try 512 or even 1024 for this experiment) you might have a similiar CPU usage in the taskmanager…
for some reasons I had projects with high CPU (asio meter) load which are “smaller” then other projects which are using less CPU… This is just a cryptic thing 
Sometimes a plugin for example only used one Core… lets say you max our Core1… have Core 2-8 more or less unused… this will already result in crackle / drop outs… in a perfect world the load would be spreaded nicely across all cores… well… in a perfect world
Thats why I have the feeling that it makes more sence to have “less cores but with more GHZ” when choosing a CPU - instead of wasting 2000 bucks for a 16 core thing… which will give you exakt the same performance like a 4 core when maxing out one core… well I am not a computer expert but that’s the feeling I have 
The newer the Nuendo Version the better the Core/CPU handling… Nuendo 5.5 was an awesome release… but currently using Cubase 8 I can tell you that here you will have a better handling of the ressources which will/might/can result in “less asio meter load” … CPU usage in Task manager should be similar…