faster spiFlashWriteByte

Of course it could be that the chip introduces additional latency when writing to a GPIO pin

Yes, that's what I mean. I believe the IO bus on-chip is 16MHz

I cannot paste in the left pane of the IDE, only in the right pane.

Works for me. Ubuntu + Chrome too. Ctrl+V ?

Actually I found maybe a better set of tests for you:

// write an empty (0xff) file of 115kB
require("Storage").write("test","",0,240*240*2);

// draw it to screen (all white)
t=getTime();g.drawImage({width:240,height:240,bpp:16,buffer:require("Storage").read("test")});print(getTime()-t);
// 0.47 on a recent-ish build
// 0.36 with most recent changes

// sum all bytes
t=getTime();E.sum(require("Storage").read("test"));print(getTime()-t);
// 0.80 on a recent-ish build
// 0.67 with most recent changes

// crc
t=getTime();E.CRC32(require("Storage").read("test"));print(getTime()-t);
// 0.63 on a recent-ish build
// 0.50 with most recent changes

More info at https://github.com/espruino/Espruino/issues/1849