I just compared IrfanView (v4.2), XnView and FastStone Image viewer (latest versions of both) with Compupic
for speed. Compupic is shareware and has been abandoned for years but none of these can match it for speed, even now.
It might lack some feature or other, making it a no-go for a lot of people but I definitely recommend taking the trial for a spin once, even if it's just to use the awesome instant zoom (tap Z) feature.
The XnView Shell extension is a keeper, BTW.