For anyone affected/interested.
Finally, I could test if the lag is present on a single CPU machine and it turns out its NOT, loading and saving files and clip files is instant - no lag whatsoever.
Then, I could test another dual CPU machine and it ALSO shows the lag in exact the same way like with my main workstation.
One of the affected PCs runs Win10, the other Win7, since the issue is present on both machines we can probably eliminate Win10 as a possible culprit.
I'd say this pretty clearly points towards CPU threading problems with dual CPUs.