a question about running integration2d_threading.py using multi-threading

[qianglise]

hi,
i was running the multi-threading example integration2d_threading.py recently, and there is something I do not quite understand.

I was testing it on Tetralith, running integration2d_threading.py
using 1 thread took about 11 s
using 2 threads took about 39 s

Why is there such a big differences when running 2 threads compared to 1 thread.
I know there is overheads, but i do not think it is that much.
Do you guys have any idea?
Thanks in advance.