- Datasheet: Up to 133mhz
- print(str(machine.freq())) = 125000000 (hz, presumably)
Have you read the various pico datasheets? If not, you should familiarize yourself with those. A microcontroller like the Pico is a different animal than a microcomputer like the Raspberry Pis. You will need to need to be a bit more hardware-oriented when dealing with a microcontroller.
The system clock speed is determined by several factors. The frequency of the crystal oscillator is the primary determinant, but not the only one. The external oscillator used for the Pico W (Y1) appears to be a nominal 12 MHz unit (p/n ABLS-12.000MHZ-B4-T) with a 30 ppm tolerance. Variances from the oscillator's specified load capacitance will further increase its frequency variance, as will the stability of the phase-locked loops (PLLs) that are used to step up the oscillator frequency to reach the system clock frequency.
There's a decent, brief description of some of the factors that influence the oscillator (and therefore the system clock) frequency in Section 2.3 of this RPi document. For further details, you can find many resources online (for example). It's a relatively deep topic; there are entire books devoted to it if you're that keen on the details.
As you've recognized in your question, the specifications for the RP2040 chip (and therefore your Pico W board) indicate a maximum system clock frequency of 133 MHz. They also specify the
"RP2040 supports 1MHz to 15MHz crystals". Consequently, one would expect the maximum system clock frequency of 133 MHz to be reached with the 15 MHz crystal. Since the Pico W board uses a 12 MHz crystal, it seems reasonable its nominal system clock frequency will be less than the maximum. Note this is not a precisely linear relationship, but in general the value of the system clock frequency will increase with increasing external input frequencies.
Finally, if you feel that 125 MHz is not sufficient, the RPi documentation suggests at least two alternatives for increasing it: replace your current Y1 component with a 15 MHz unit, or replace it with a CMOS clock source whose output is a 3.3V square wave.