The Up SideThe RF24 (nrf24l01, nrf24l01p, nrf24l01+) radios use the SPI protocol to communicate with its master device. The speed at which the devices communicate is controlled by the master's clock speed. Here, the Arduino is the master device. Accordingly, we can control the Arduino's SPI bus speed. On the Arduino, this clock speed is set by means of a divisor based on the speed on the CPU. For a standard Arduino, the CPU runs at 16Mhz.
To reduce the chance of communication errors, a compromise was made in the RF24 driver to not run at maximum bus speed. By default of the driver, the divisor value is SPI_CLOCK_DIV4. This divides the 16Mhz clock by 4. Which drives the SPI bus at (16/4) 4Mhz.
To further improve performance, it is possible to use a divisor of 2 (SPI_CLOCK_DIV2), allowing for an 8Mhz SPI bus speed. That's twice the performance of the default setting. This translates into half the latency for SPI bus communication. If you're pinched for maximum performance, this may well be a change worthy of consideration.
To make this change, look at RF24.cpp. Find the line containing "SPI_CLOCK_DIV4". Change that value to one of the other supported constants to adjust the SPI bus speed accordingly. Just remember, its a divisor, so the larger the number, the slower the SPI bus speed. Inversely the lower the number, the higher SPI bus speed.
The Down SideThere are down sides to running at faster speeds. And this is why a lower value was picked for a default. At faster speeds noise becomes much more noticeable and communication between devices becomes more easily corrupted. Corrupted communications means packet loss, an increase in communication errors, and/or corrupted packets at the receiving end of things. This is important because, should a packet become corrupted during SPI transfer, it will be delivered in its corrupted form at the receiving radio. In spite of corruption, it will still pass a CRC check because it was received exactly as was transmitted - corrupted. That's not to say these things will happen. But they can happen. So caution is advised.
For breadboard use, when jumper leads are long and noise is likely, I strongly encourage you to maintain a slower SPI bus speed of 4Mhz. But, once you're ready for production use, where you have everything connected properly with decoupling capacitors and short leads, a clock divisor of 2 (SPI_CLOCK_DIV2), providing for an (16/2) 8Mhz SPI bus speed, may well give your performance a tiny, extra boost.
CustomizationAs happens with many Arduino projects, you may find you've simplified things by converting to a simple AVR project. Many users who do this deviate from the 16Mhz clock speed to something slower or faster, as dictated for the given project. As such, if you find you've increased your clock speed to 20Mhz or even decreased to 1Mhz, don't forget you can tweak these settings as needed. Always remember, SPI bus speed is determined by the speed of your CPU. If you change the speed of your CPU, you might re-evaluate your SPI bus divisor.
History & SummaryWhen the original SPI bus speed was selected in Maniacbug's code, I did observe errors when running at 8Mhz on a breadboard. Infrequent yet present. This is why I picked a 4Mhz speed as a default value. That, however, doesn't mean you can't try faster.
Be mindful of your decision should you decide to run at a faster SPI bus speed. Use leads as short as possible. Use decoupling capacitors as close to the radio as possible. If you're unsure, leave it where its at. Worst case, you now know you have some options to squeak every last bit of performance out of your project.