Such an error is usually due to the presence of a RadToolTIpManager with AutoTooltipify set to true. It has to go through all elements on the page and attach handlers to the needed ones (elements with title/alt attribute), so when you have a very large grid (e.g. 500 rows and 20 columns which results in over 10 000 elements just from the grid) and a very slow browser (like IE8) it will take a lot of time for this operation to complete. There is no possible workaround for this apart from reducing the page size, because such automated operation will always come at some performance cost.
In case you can confirm you have no tooltipManager, this problem would be rather strange. Standalone RadToolTips also attach handlers to their targets when the pages loads, but for a performance problem to occur you would need thousands of tooltips so that the event handler addition becomes noticeable. If the sheer number of controls is the case reducing the page size would be, perhaps, the easier option.
What I suspect is a problem is the fact that you add <html> tags in the label, which is invalid, so you can try removing them to see if this helps.
I cannot really be sure what is happening because there is no target defined for your tooltips in the code snippets you pasted, so I can only guess.
On performance optimization for RadToolTips I can suggest another approach, however. You can use the client-side API of a RadToolTIpManager to dynamically create tooltips when mouse events are fired for their targets. This will let you avoid having several thousand event handlers added at the same time on page initialization: http://demos.telerik.com/aspnet-ajax/tooltip/examples/radtooltipmanagerclientapi/defaultcs.aspx
. If the targets have a title attribute set it will automatically become content for their RadToolTip, so you do not need to define a load-on-demand scenario.
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