Originally the trackball and tooltip behaviors were created with the intention that they would be used when the chart is not that busy. This is because when there are so many items the trackball/tooltip can provide only a very small piece of information compared to the whole picture (you will see trackinfo for a single point when there are 20 thousand points). This is one of the reasons that both the trackball and tooltip were not optimized.
However, we have seen many customers using the two behaviors in busy scenarios and we have identified the need to improve the performance in such cases.
Unfortunately, currently you cannot override the FindClosestPoints method, because it is internal. I think that the best way to go here is not to use the behavior. If you know your items are sorted, and you can find them very fast - simply implement your own trackball-like control. You can place it completely out of the chart, on top of it. You can use the PlotAreaClip property of the chart to find where exactly you can position this control.
Let us know if you need more information.