For the fastest built-in option on macOS, use Quick Look. It’s lightweight, opens quickly, supports full-screen viewing, and can browse multiple images from a folder. A common workflow is to select the images in Finder and open Quick Look in full screen, then move through them with the arrow keys. It also offers an index/contact-sheet style view for thumbnails.

For zooming, macOS provides a built-in screen zoom feature you can use while viewing images, though you may need to enable it in System Preferences first. If you want a simple app instead, Preview is also a solid choice for quick image review.

If you prefer a dedicated lightweight image viewer, Xee was recommended as a good option that sits between very basic viewing and heavier photo software.

So: Quick Look is probably the best first choice since it’s built in, with Preview as a simple alternative and Xee as a lightweight third-party option.