Why is it so much faster than the built-in viewer?

Because it does less. There is no runtime to start, no library to load, and when opened by association it creates only the full-screen window — the file shell is built only if you ask for it.

Which formats does it read?

Everything the Windows imaging component supports, which covers JPEG, PNG, GIF, BMP, TIFF and the camera formats your system already knows. Nothing extra is installed.