800,000 tracks! What an impressive collection you’ve got. That’s in the top 0.0001% of all Roon subscribers (and I’m probably missing a few 0s). 
Keeping that in mind, the behavior you’re describing sounds about right. You’re pushing Roon and your hardware to its limits and seeing a maximum of 6-7s loading time only in certain cases. It’s not ideal, but with a collection so large stored on NAS, it’s not a bad result either.
If I could suggest any improvement, it’s to remove the NAS from the equation and go for directly attached storage. Our CTO’s post below lists a number of advantages that come with directly attached storage.