The PIC18 series have a maximum internal oscillator frequency of 8 MHz, which translates to 2 MIPS. However, the PIC can run at much higher speeds (especially when using an external clock source). Using the PIC's PLL, one is able to multiply the clock source by up to 4 times. This is even software-controllable if using the internal clock source. Using the maximum speed of the internal oscillator, the PIC can run at up to 32 MHZ with PLL enabled. This yields blazing fast performance of 8 MIPS. Best of all, you can turn this feature on or off as you please to control power consumption. Using the PLL is very easy.
We must first ensure that we are using the internal oscillator. So, in C18, declare a configuration option like this:
#pragma config OSC = INTIO67
Next, we must either configure the internal oscillator to run at 4 or 8 MHz. The PLL will not be enabled on any other configuration. Since we're opting for speed here, let's set it at 8 MHz. Put the following as the first line in your main() function:
OSCCON = 0b01110000;
Note that the last two bits of OSCCON must be 00. And finally, to enable the PLL, add the next line right after:
OSCTUNE = 0b01011111;
This enables the PLL and runs the PIC at the maximum possible frequency using only the internal oscillator - 8 MIPS @ 32 MHz. To toggle the PLL on or off, simply set OSCTUNEbits.PLLEN to either 1 or 0, respectively. Refer to the datasheet for information on how to fine-tune the PLL.