Heureka, I could solve it...important for EVERYONE who determines slow loading times. Since I'm working on projects in Cubase that easily use 50GB of RAM, I've currently set the buffer size of my sound interface to 1024 samples (to avoid CPU spikes). As a result the performance drops depending on the Kontakt instrument as well, as we can see with Lunaris 2 and Bioscape. Setting the buffer size to 521 samples, Lunaris 2 loads twice as fast (loading takes about 4s), 256 samples Lunaris 2 takes ONLY 2 seconds! That would make the system as fast as hell again. Finally it seems that only Kontakt instruments are affected if running a script when started, loading pure samples such as Spitfire Symphonic Strings etc. do not affect the performance, regardless of the buffer size.
I still need to try this (currently performing a fresh install of Windows 11), but that would make sense given that I usually set my buffer size really high (at least 1024, if not higher) due to the heavy loads of my projects. Thanks for the info though 👍.