Jpgs open almost instantly.
So let's check some of your settings.
Click on VIEW Menu at the top.
Make sure there us a check in jpg so Qimage knows what to ignore .
See snap.

Thumbs quality set to BEST

Thumbs ON/ OFF set to ON

Thumb size Large

Select as many thumbs as you wish by highlighting them and holding the control key.
Now click in that same menu, Rebuild Thumbs.
Results are???
Fred

OK Bear in mind.
Refresh thumbs restores thumbs only. Rebuild thumbs tells Qimage to rebuild thumbs and CACHE FILES.
Qimage will generate a special image file for each image which it uses internally. That will take longer, depending on the size of the image. (number of pixels).
There should be no concern for the user as this cache build takes place only once.  If you make any adjustments or refinements to the image, of course, Qimage will need to make a new cache and a new thumb reflecting the change to that one.

Another item to check is the setting for your processor to be set to most efficient for this intensive process. Please see screen snap.
Last one would be to keep an eye on the progress as the thumbs and cache are building.
See screen snap.    It shows the number of images to be processed and the  progress     45, 46, 47,  etc. as it moves through the folder.
See if it gets stuck on any one particular image. We root that one out, and all good.

Fred