Complexity of an app

Hi Forum

Can somebody tell me what’s the complexity of an application when I use an incremental grid?

i.e, if I execute an app with

1000 threads it takes 2 seconds,
2000 threads it takes 2.x seconds,

1 million x, seconds.

And also I would like to know why an app with different quantity of threads takes different execution times.

It is assumed that 1 threads should spend the same time that 1 million of threads, no?