It may help. To get any real benefit, though, you'd probably need to defrag your hard drive after you'd deleted them. It's got to be worth trying, anyway, because even if it doesn't speed up your PC, you sure as hell should have copies of your photos. I'd strongly suggest two copies before you delete anything. Note that, depending on the size of your images, you may need either a very high capacity USB stick, or several of them.
If it's an oldish PC, though, and you've never reinstalled Windows - reinstalling Windows may be the way to go (after you've backed up all your files).