The Arduino Serial writes one byte at a time. There is a method for
writing a buffer of arbitrary length, but all this method does is
repeatedly call write(uint8_t) for each byte within the
buffer:
size_t Print::write(const uint8_t *buffer, size_t size)
{
size_t n = 0;
while (size--) {
if (write(*buffer++)) n++;
else break;
}
return n;
}
Notice that there is no waiting between the bytes.
If you do not fill the transmit buffer, this should be very fast: all
write(uint8_t) does is push the byte into a ring buffer. Unless your
timings are at the microseconds level, you can just send the six bytes
at once.
I've heard the Serial library processes 16bits (2bytes) at a time?
Very dubious. Do you have a reference for this statement?