This means that there are a lot of elements (nodes) that need to be handled on the client which can cause slower browsers like IE7 and IE8 (especially when used on slower machines) to not meet all your needs.The charts will, however, continue to function properly and show the data, it is just that the browser does not manage to render them fast enough.
In addition, when the charts have their animations enabled (which is true by default) the properties of the multitude of nodes mentioned above must be changed rapidly. This, in turn causes the browser to re-render the image, which is an expensive operation. Now, when you multiply this effect five or six times for example, this will play a more significant role in the decreased speed of the rendering.
What I can suggest you in that case is that you simply remove the animation of the charts by setting the Transitions property to false. This is not a bug/issue in the control but a browser behavior in older ones that are simply slower.
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