Doubt on Processing Power
Processing Power is a bit of a misnomer. Clock Frequency is a better description.

In general one assembly instruction gets executed each clock cycle. That means approximately 16 million instructions per second (there are a couple that take more than 1).

Your C program gets compiled into those instructions and executed at 16 million instructions per second.

If your routine is 16 million instructions long (when loops are unrolled) then it will take 1 second to execute.

Unlike a PC, which is constantly switching between tasks, an Arduino does only one thing, and that is the one thing you programmed it to do. Because of that execution time is predictable. If it took 1 second to execute today it will take 1 second tomorrow,  and 1 second next week.