Design for Speed
For a given power, speed is increased by reducing the total drag. At high speed, you want an airfoil with the lowest coefficient of drag at lift coefficients between zero and about 0.2. Low wing loading helps keep the lift coefficient low and therefore the induced drag low. This is vital in a low aspect ratio configuration like a delta.
The dominant item of drag in the drag budget of models like this is the parasitic drag so streamlining and cleaning up the airframe is the number one priority. Cowling engines, enclosing pipes, fairing spinners into the fuselage and, putting switches, jacks and fuel filler fittings behind hatches are some things to do.