I had to edit my posting because i used an “older” version of GP, but it’s the same effect with the new V3.8. 
At his point, i’d like to remind everyone of the “inner circle” that we are discussing this issue in the public area of the forum!
I reduced the number of plugin blocks to 4 in each instance and everything runs smooth now.
The network traffic and the number of used tcp connections have decreased significantly and there are no remarkable CPU-peaks anymore.
@schamass - Thanks. I suspected that those plugins were doing something that puts the refresh rate at a threshold. I’d still expect no glitches in the actual performance though 
If you map a plugin parameter to a widget, no values are sent to the plugin for the “freeze” moment. Which is of course bad. But midinotes behave normally. No notes are stuck.
I don’t have any Native Hardware either. I use the Native Komplete plugin to tag, manage and categorize all my difference plugin sounds. And finally to load them.
It’s not just that widget that hangs in those moments. There are also widgets hanging that I have assigned with a midi controller. No values are sent to a mapped plugin parameter at these times either. It also hangs the complete GP GUI for those short times.
Strange thing is: If you have e.g. 40 “Komplete Kontrol” plugins in only one GP instance, the problem does not exist.
If I e.g. replace the 16 Komplete Kontrol VSTi of an instance with 16 randomly selected other plug-ins, everything runs fine without stopping. Even if they are “big” plug-ins and the CPU usage is over 60%.
However, this would be contradicted by the fact that the use of e.g. 40 “Komplete Kontrol” plugins in only one GP instance (and only one rackspace) would work fine, right?
Edit: The error also happens when all 32 “Komplete Kontrol” plugins in both GP instances are on bypass.
@schamass Thank you very much for your help and analysis. And yes, this happens with all GP versions. Even with the very latest one.
I am already reassured that it is not a problem of my computer configuration. But it seems to be a Windows thing, because @pianopaul does not have this problem.
The way I got to know the support of Native Instruments, you can’t expect a help from their side. I stupidly spent a lot of time to manage all my different Pluin sounds with “Complete Control”.
That’s why this is so important to me, and I hope that GP will be able to fix this problem, since it only occurs when two instances of GP use the plugin.
Finally I have tested the following:
- 16 “Komplete Kontrol” plugins in one GP instance
- Simultaneously Steinberg Cubase with 16 “Komplete Kontrol” plugins
-> GP and Cubase “freeze” again and again. Obviously a problem of Native Instruments not GP!
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@LeeHarvey yes - I wanted to suggest something similar next
Thanks for confirming. It may be related to the fact that @schamass found out about networking and if the plugins run in different apps they behave erratically …
I even didn’t know that you can use it without Native hardware and thus never used it. Perhaps I should, but only in one single instance 
On Mac no Problem