It won't import them as new files, but will update the metadata including IPTC and adjustments. I suggest you try it with a small number of images and work through it a few times.
Snappiness is rarely related to the number of pictures in the catalogue (I work with some containing hundreds of thousands), but the backup is one of the few reasons which can justify working with more than one catalogue. Even then, a system backup should be timed for when you're not working in LR, so you're not losing any valuable time by backing up one large catalogue (you aren't backing up the previews, are you?). You'll be wasting a lot of time moving images out and in, and you also risk confusion from images being recorded in two catalogues. So when people insist on breaking up their workflow, I generally recommend an "inbox" approach where you only import from card into a temporary catalogue - call it "new shoots" or "new work". When you feel you've finished working on the new pictures, then use Import from Another Catalogue to bring everything into the main one, and archive/delete the inbox.
Make sure you review http://kb2.adobe.com/cps/400/kb400808.html