Look at the picture. It is my FPS measured with AVSMeter and latest version SVP4.
I have 4 threads CPU and Auto threads for it was 7. I see the maximum FPS is for threads=10. It is surprising for me because I see steps on diagram but they grouped by 3 not by 2 as was earlier: threads=1,2,3; threads=4,5,6; threads=7,8,9 and so on.
And yes. 7 threads is good enough value for threads=Auto.