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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.
@Gordon
I think you really need to stick an oscilloscope on it and check what's being output.
I believe the F4 chips actually execute more instructions per cycle than F1s in some cases (Cortex M4 vs M3) so that could be knocking your timings out.