I recently updated to Kontakt 8 and it's been more trouble than help. I work in Reaper on Windows 10. I do not do multi-out, so I have individual Kontakt Instances for each track. This has always worked fine for me with no performance issues.
I recently went through my main template and started changing my Kontakt 6 instances to Kontakt 8. This is when I started noticing the problems. Each FX window with Kontakt 8 loaded into it would take 1 to 2 minutes just to close out when I was done with it. Not only that, when there are any Kontakt 8 instances on the track, the project save time takes anywhere from 3 to 4 minutes. This was unreal and unlike anything I'd ever experienced.
I tried with multiple libraries and different sized project files. The issues were still present in projects with fewer instances overall, but it was still slow nontheless. I reverted to Kontakt 7 player and everything started running like a well oiled machine again.
I am extremely disappointed that this issue exists and I am tempted to ask for a refund on my upgrade since it is basically unusable. It is unacceptable for a $300 sampler to have this sort of issue.
