Uint8Array - is it really an array? And what about the other Uint#Arrays?

If you want data out of SPI as a string, just give it a string to start with - that would be the fastest method :)

I've just fixed Function.apply - so 1v72 will now work - although the maximum number of arguments (and hence characters) is still 64.

Is string concatenation not the recipe for increased garbage collection activities

Actually no - because Espruino is reference counted it's actually very good at stuff like that - and the blocks of memory also make concatenation much more efficient than you'd expect :)