There are many characteristics to consider when optimizing for strength. Obviously, this is highly dependent upon the material properties of your filament. It is also highly dependent on the number of shells in the part, the infill shape, and the infill density.
To optimize for Speed on a German RepRap printer, you need to consider the layer size (similar to optimizing for surface finish...only opposite). In this case, the nozzle diameter is also important to consider since we cannot print a layer thicker than 80% of the nozzle diameter.
We can optimize surface finish on any German RepRap 3D Printer with Simplify3D (our slicer software). Surface finish is a function of layer size and nozzle size. The smaller both are, the better it looks. The tradeoff is print time, the smaller the layer and nozzle size are, the longer it takes to print.