Microcontroller clock speed vs internal oscillator
The midrange series PIC chips generally run at a clock frequency of the external crystal divided by 4. There are 4 t-states in each cycle, full details can be found in the reference manuals. Each instruction takes one or two cycles.
So an external crystal of 20MHz will result in a 5MHz (200ns) cycle.
The internal RC oscillator can run at up to 8MHz (no pins used, no external parts but very poor accuracy compared to a crystal or even a resonator). At 8MHz you will have a 2MHz clock rate at the CPU.

These 8-bit midrange PIC chips are extremely simple. Modern 32-bit MCUs often have PLLs and fractional PLLs as well as dividers that can generate a huge range of frequencies from one crystal or external (or internal) source. This becomes more and more necessary as you need things like Ethernet, I2S and so on that each demand different frequencies at tight tolerances.