Is the UI Virtualization of the RadGridView enabled on your side? You can take a look at the UI Virtualization
article in order to learn more about that. Note that the virtualization is enabled by default.
Keep in mind that the virtualization won't work properly if the RadGridView is placed in a control that measures its children with infinity such as ScrollViewer, StackPanel or Grid with RowDefinition Height="Auto" or ColumnDefinition Width="Auto".
You can also read the Tips and Tricks
article to learn about more reasons for the Virtualization to be turned off and potential approaches for optimizing performance. Another thing you can check out is the Degraded Performance
Can you give these articles a try and let me know how that goes?
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