This sounds sort of innocuous... plain vanilla something or other, but it isn't.

After Ticking the Preview box, or double clicking the selected folder;
Obviously, the new scroller in the search screen is a great aid to scrolling through the thumbs of a folder while leaving the search screen open in case the images you are looking for are not in the first folder.

Now you can select folder two, and scroll through there...

Data BASE search:   This one I love.
I have "stuff" all kinds of images from as many cameras that have crossed my path over time in a collection folder.
I have Raw images from ARW  and RW2, to all kinds of NEFs and CRWs... from Olympus, Sony, et al.
I keep these so I can simulate some situation as closely as I can to help someone with an issue.
So now I need to find some ARW images from a Sony A7 camera.

I do a FILE/FOLDER search in the selected folder or folders (you set the ground rules) for ARW.
Ultimate's search found the ARW images in a folder that has 400 images.
OK, the thumbs from the ones I want have been selected,,, blue framed, but really? 400 to look through?

Easy peasy!
Just look above the right corner of the thumbs; see two RED PLUS signs.
If you click the one on the left, it will add the selected images to the queue. The right hand Plus is for preview before adding to queue.
See snaps.

Mike also really made a noticeable improvement in thumb/cache build stopping and starting when you open or change old folders that have no existing thumbs.
Most speed improvements are internal and barely perceptible. This one I can see!!

Enjoy...
Fred