For me, dragging to select thousands of photos is frustratingly slow.
Windows Explorer
The easiest way I've found to delete or just move a bunch of photos from your phone is simply to use Windows Explorer:
- Plug your iPhone into your PC with a USB cable
- Unlock your phone and, if it asks, allow the PC to access the iPhone.
- Run Explorer (Windows key+E is one, quick way)
- Select This PC
- Double click on Apple iPhone.
- Double click on Internal Storage.
- You'll now see a DCIM folder. Go in there (by double clicking it).
- You'll see some number folders. These are where your pictures are.
- Move or delete your photos. Be careful here. These are not going to go into your PC's Trash nor the iPhone's Recently Deleted when you delete them.