> How did you arrive at this number?

It's in the post. Half a cycle for an add or less, and cycles are every 1/3 nanosecond. So upper bound for an add would be around 1/6th a nanosecond. Likely less than that still yet, since the M1 is probably closer to an add in 1/8th a cycle not 1/2. Skylake by comparison is at around 1/4th a cycle for an add, and since M1's IPC is higher it's not going to be worse at basic ALU ops.

6 nanoseconds @ 3ghz is 18 cycles. That's on the slow end of the spectrum for a CPU instruction.

Where? 6 nanoseconds is pretty long, that’s about how long it’d take to do an entire retain/release pair, which is a couple dozen instructions I believe.