It shouldn’t be about the difference between Add or Copy. Sometimes it’s about the preview it needs to generate for the imported images. For the fastest import, in the File Handling panel of the expanded Import dialog box, set Build Previews to Embedded & Sidecar. Then it will not spend the time to render its own previews.
Import will probably take the longest if Build Previews is set to Standard or 1:1.
Possibly becuase LrC is doing a lot of work in creating previews when you import? Possibly because your computer is slow? How long (in minutes) does it take to import? How many photos are you importing? What camera (make and model) took these photos?
I can't really know the reason without more explanation of the problem and the information requested above, and without knowing your computer hardware. Please provide (as text) the information in Help->System Info.
So if you have a file in Folder A, and you select ADD, the file remains in Folder A and is imported into LrC.
If you select copy, the file is copied from Folder A to Folder B (whatever folder you specify) and then LrC imports it from Folder B — so LrC only know about the photo in Folder B, it does not know about the photo in folder A (which is still there)
p.s. i don't think the adobe website, and forums in particular, are easy to navigate, so don't spend a lot of time searching that forum list. do your best and we'll move the post (like this one has already been moved) if it helps you get responses.