I'm very sorry, may I did not describe my problem clearly , now describe it.
In IE browser, in a more content page, chart is at the bottom of the page,
chart rendering is completed, by dragging the scroll bar down, chart is normally seen in the screen.
And then drag the scroll bar up until the chart is not visible on the screen,
in this case, call "chart.exportChart ()" for server-side export on the javascript,
exporting is very slow, it takes about 3 minutes.
And in firefox browser, in the same situation exporting is faster.
How to avoid the problem in IE browser, thank you very much.