Thank you Shenghui Yang for your help.
I'm not expert in mathematica but if your solutions improve a lot the speed, they do not produce the same result, unfortunately.
if you make a zoom around 0 you will see the that the plots with your methods gives much less details than the original plot...
I changed also mathematica 10.0 to 10.3 and the computation time went from 81s to 67s without changing a line of code....
then, it's already better... but v9.0 is still better....
