'Optimizing' is when DS converts the surface info into 3Delight's prefered .TDL format, these .TDL files are temporary files and get deleted when DS is closed.

When importing Poser content DS automatically optimizes the surface info in the background, and it's finished almost as soon as the files are loaded, but on loading a .DAZ scene file it can take it's own sweet time before it starts, if you hit render as soon as the scene is loaded you almost always get hit with 'Optimizing'.