There's only so much optimization that can be done. The program needs to snap a screenshot of the screen, compress it to something small enough to send quickly via Wifi, and then the DS needs to be able to uncompress it quickly and display it. There are tons of bottlenecks that can cause slowdowns. My suggestion is to lower the settings on the PC app, then on the DS disable one screen entirely and set the other to update at "Ultra" speed. That's about the fastest you'll be able to get.