Hello GrinGEO,
Vertex color calculation option doesn't result in considerable time require additionally to the surface generation itself. These colors are applied to each vertex basing on the average colors of the corresponding areas on the source images. However, the size of the exported mesh file will be larger, if colors are saved (but it can be disabled in the Export dialog)
If you are working only with the surface shape itself or with the applied texture, then you probably do not need these colors.