We are using ogre 3d for our game and there has been some debate about whether normal maps slow down our game, so we’re bringing it to you guys. We are making a MMORPG so need all the speed we can get.
Normal maps don’t slow down a game nearly as much as the geometry necessary to get the same amount of detail, and I think any slow down is usually worth it.
It also depends on your style. If you are going for a more realistic look, I would say you almost have to use spec and normal maps. On the other hand, if your style is more toonish, you can get away with just a diffuse map.
Like Kupoman said, normal maps don’t slow down a game near a much as the geometry required to get that detail.
They do have an effect though, so if you can’t tell the difference between having them on and having them off, remove them. Once I had a normal map on an object. It ran about 40fps. As soon as I removed the normal map it went up to 60fps. Most of the time the speed won’t be very noticeable, but it can be.
I would say make them, and add an option to enable them, so faster GPU’s can take advantage of normal maps, while people with slower computers can still play with a decent framerate.