Adding modules into the existing PX4 Autopilot architecture can be quite challenging, but does not require extra hardware. The Kinetis K66 on the RDDRONE-FMUK66 is a fairly powerful microcontroller, but you cannot compare it to the processor in your personal computer. Resources (RAM, processing power) are limited and your own modules have to share this with the rest of the PX4 Autopilot software. When building more advanced applications, you might have to learn more about the PX4 software architecture and embedded software in general.