feh works very differently wrt eog. It pushes the (suitable: zoom/part) image as bg pixmap of the X window. So X server keeps it, and feh does not care about Exposes. This implies more memory consumed in the X server. Since I find its zooming (via pointer movement, while button2 is down) buggy, I cannot make a clear idea on the comparison of the speeds of zooming.
merriam: feh's zoom is quirky. You set the centre of magnification with the button2 down. Perhaps you know that already, but it's far from obvious. Then you can run out of screen while zooming in.
I find it slower (see artifacts) when dragging a non-completely visible (zoomed) image (dragging to expose other parts) than eog