As the mentioned article states - the chart' image is created by numerous XML nodes that represent chart's elements. The more chart's elements are presented the more XML nodes will be rendered and therefore the page load time will increase. Therefore the mentioned behavior of the browser is expected if it has to render a RadHtmlChart control with 40-50 thousand items.
Note also that the performance greatly depends on the performance of the machine and the browser. For example slower browsers like IE7 and IE8, especially when used on slower machines, may not meet all your needs.
All that being said I can suggest that you decrease the number of the chart's items.
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