Just wanted to inform you, that I just had the issue that two different plugins used together in the same project caused CPU spikes after freezing tracks or doing a mixdown.
Only a restart of Cubase then solved the issue.
By updating the plugins the issue went away (It was an out of date version of Decapitator used together with Sand3 from Acustica). When only one of those plugins was used in the project all was good.
Just to show you that even if you sorted out different plugins as the cause of your problems, in combination they can still f*ck up your preformance.
Took me an entire day to troubleshoot this.
This again brings me to this FR: Solution to crashes by plugins - Sandboxing (Crash protection) - Cubase - Steinberg Forums