Speeding up Thumbnails?

I think I have the latest V9.0.2.219 for Windows.

I have several large libraries that need to be visually searched with thumbnails.  I am currently opening a 50 GB folder with subfolders and approximately 10,000 files, probably 80% of them graphics files.

The files are stored on a remote cloud drive, so there is some time delay in retrieving the full files.

It now looks like it will take 5 plus hours to build the cache of thumbnail previews. 

Is there a way of speeding up the building of the thumbnail cache?  Possibly by suppressing the creation of 100% previews completely?  I have already checked the box to prioritize thumbnails over previews.

Further:  I just want to confirm that, once the thumbnail cache is built, it will remain until purged. 

Is there any problem, given sufficient disk space, in maintaining the cache without purging it?