read an high precision adc like hx711

Ok, use hardware SPI - it should provide a steady train of pulses. Instead of:

var s = new SPI();
s.setup({ miso : data_in_pin, sck : clk_out_pin });

do:

var s = SPI1;
s.setup({ miso : data_in_pin, sck : clk_out_pin, baud: 1000000 });

you'll just have to choose pins like B3 and B4 that work on SPI1: http://www.espruino.com/Pico#pinout