The graph represents 1, 2, 3, 4, 8 threads and I have 8 cores.

Actually I haven't thought about the time required to spawn the thread... since each thread has at least 55k sets of 7 values to work on, I thought the time required to spawn the thread was negligible.

1 thread: 60 seconds
2 threads: 40 seconds
4 threads: 37 seconds
8 threads: 54 seconds

I think I will settle on 2 threads for the time being, it should be a good compromise also for smaller datasets.