I think I just answered these questions with my replies in the branch just above.
However, it is important to understand the mechanics of PM.
When you open a folder or multiple folders in a PM contact sheet it has to read at least part of every image in the structure. This could be thousands of images that consume gigabytes of disk space.
PM is smart about how it does it. For example, it only reads the metadata and the embedded thumbnail. If it read the entire image file it would take forever. PM is probably the fastest browser around, when it comes to this initial read. And it reads ahead in a smart way in order to give you the perception that it is reading fastger than it is.
But there are limits because this is necessarily a brute force method.
The solution to this problem of scale is an image catalog app. In that case, the app stores thumbnails and all the metadata a single database construction that is searchable.
More accurately the cataloger will show you a folder structure of keywords. You click the keyword of interest and it returns the images in some sort of contact sheet or thumbnail window.
Another interesting and unique feature of iMatch is its "offline cache". This is of interest to those (like me) that work primarily on a laptop, and access their catalog from that laptop.
My image repository is located on a dedicated machine acting as a file server. My iMatch database and the "offline cache" is located on my laptop.
The database file (a single file in the case of iMatch) contains the metadata and a small thumbnail, typically about 300 pixels wide (although in principle it could be any size you want but be careful what you wish for).
My iMatch DB is about 10GB, and contains about 300,000 images.
The offline cache contains a low res image of, in my case, 1400 pixels in size. IN theory it can be as big as I want it to be but it quickly approaches the size of the actual image repository so it has to be reasonable. The cache is composed of jpg files, one for each image. The folder structure and image naming of the cache is only usable by the iMatch app.
My offline cache is about 30GB is size, making my entire iMatch database stgructure about 40GB. That is a very doable size on a modern laptop. It would even easily fit on a fast SSD drive and there would be much benefit to that.
When I am home and connected to my network (usually hard wired), I directly access my original images from iMatch. When on the road I access the low res 1400 pixels cache images, which are far preferable to tiny 300 or 400 pixel thumbnails typically stored in a main catalog database.