Over/Underclocking?

Ok, just committed, so you should be able to download one of the Git builds in an hour or so and do the following (on a Pico):

// Do nothing (hopefully!)
E.setClock({M:8, N:336, P:4, Q:7, PCLK1:2, PCLK2:4})
// Halve clock speed to 42Mhz, keeping UART/etc working fine
E.setClock({M:8, N:336, P:8, Q:7, PCLK:1, PCLK2:2})

You can do loads of different stuff, but you'll have to look at the chip's reference manual to get the correct values.

You can also experiment with changing the flash latency with latency. At lower clock speeds you should be able to get away with a latency of 0, which will make the processor a bit more efficient.

The STM32F1 in the original Espruino board is completely different clock-wise, so I haven't included anything for that yet.