Was LR CC ever tested with large libraries?

It would be interesting to know what options you've set on the Preferences>Local Storage tab. Could you post a screenshot of that tab, maybe we can make some suggestions to help improve the performance.

Thanks for sharing your experience with us. I’ve wondered myself how Lr CC would handle a very large library. I also wonder how large a library Adobe has tested it with. Of course internet speed will be a factor for uploads and downloads. I’m assuming you have good, dependable internet. Not everyone does. It’s interesting to me that you find the web connection the fastest. I find Lightroom web is really slow to enter the edit mode, but everything else seems pretty responsive. I’ve only tested it with a small number of images though.

There are two different Lightrooms for two different types of users. In my opinion (and this is only my opinion), Lightroom CC is not designed for someone who has a large library such as yourself that needs careful management. Lightroom CC is designed more for the user who wants quick access to images across multiple devices, and a quick way to have those images shared via the web. If you have a huge library that you want to carefully manage then I think it would be much better to use Lightroom Classic CC where you have much more precise control over where the images are stored and how they are organized. Additionally, at least presently the editing tools are much more robust in Lightroom Classic CC.