Thank you for your quick answer.
It is not easy to share my code because I have a complex server back-end.
1. OS = Windows 7 x64
2. Browser = Google Chrome version 52.0.2743.116 m
3. Actually the problem happens when I put the mouse over data point of the series, then if the legend is displayed and then I pan, it is slower than if I pan without putting the mouse over a given data point. It seems that the problem also existed with the version 1.8.0 but it is a little bit faster.
So this is not a major issue. Sorry about that !
Thanks !