worldx() and worldy() - pretty slow?

[Crow]

The conversion from screen coordinates to woorld coordinates via worldx() and worldy() seem to happen pretty slow. For example, when I'm showing an image on the tile layer, but its supposed to look like its on the screen layer, it would lag behind if I move. This is not the case btw.
Though, it would be useful if there was some other way that wouldnt be so damn slow and laggy. To be certain, I actually need something that returns the level x and y at the edges of your screen. Any ideas?