Hello,
OK, so we took another similar model MacPro and installed it from scratch. Installed Nuendo 5.5.6 and just after I fired it up it started to use 10-15% on idle. So this is not hardware problem or corrupted software/settings problem.
Did the install include an OS install or just Nuendo?
You haven’t installed the Contour Shuttle Pro drivers have you? Eliminate as many variables as possible and start from a wiped drive, disable the usual MacOS junk, install your audio drivers, then Nuendo.
Where are you reading 10-15%, in Nuendo’s cpu meter or in Activity Monitor?
What era Mac Pros and what MacOS version? Two of ours have been on 10.8.5 forever and work very well with Nu 5.5.5 and 5.5.6.
What audio card?
I’m assuming you have latency settings set to something normal and not low.
Hugh