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Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
@Manxome
Here is a discussion of F401 clock speeds out of the box.
If you take a look here, you can see two crystals (X2 & X3) to the left of the STM32F401 chip.
If you take a look here, one of those crystals is missing.
My F401 Nucleo board does NOT have either of those crystals.
My F411 Nucleo board only has the upper black (X2) crystal.
84MHz divided by 5.25 is 16MHz. Perhaps new boards are crippled? I am a programmer, not a hardware guy, so if I am completely out in left field...be gentle :)