How many Arduino instructions can you execute in that time frame
4800
remember every Arduino instruction is a layer of C code underneath
Some are 1:1.
So, a bit read and a conditional jump.
Two instructions, three cycles, 187.5 ns.
A shade under two tenths of a millimetre at 900 ms-1
Edit: Sorry, bit test and unconditional jump, up to five cycles, 312.5ns, so just short of three tenths of a millimetre.