If you are using Autodesk 123D Design to create your models, it will generate your STL files in ASCII STL format. XYZware is slow to load large ASCII STL files. Slicing an ASCII STL file can take a very long time too.
Fortunately, there is a solution. Saving your STL files in a Binary format will result in improved load and slicing times.
Converting your ASCII STL files to Binary is very simple. This simple guide shows you how.