I am afraid that currently there is no fix for this flickering.
Basically, to decorate controls, RadFormDecorator hides the decorated controls when the page is being loaded and then show the decorated ones. This is not happening with fieldset elements however.
If we apply the same logic to fieldset elements, a lot of side effects occur - note that the fieldset can contain complex setups including other controls that should be decorated as well. That is why the current logic of RadFormDecorator states that the fieldsets will not be hidden and they will be skinned only after all controls that they include are decorated first - this is where the flickering comes from.
I hope this explanation helps.
the Telerik team
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