Not so - this routine actually takes a long time by comparison, and I've no doubt the execution will be shamefully slow :) To push 16 bits to the hardware stack

Not so - this routine actually takes a long time by comparison, and I've no doubt the execution will be shamefully slow :) To push 16 bits to the hardware stack on the standard 6502 takes 8 or 9 cycles (depending how you retrieve the second byte); this routine takes 50 (as a round trip including the JSR).