Hi Christian Menhofer,
The CellFormatting event is fired whenever a cell changes its visible state. Therefore, when scrolling your grid, this event is fired many times for each cell that becomes visible even for a moment. This may lead to slower performance.
Instead, I would suggest traversing RadGridView's Rows and Cells collections where you can implement your logic. Alternatively, I would suggest setting the image content before binding the grid, instead of making changes at runtime.
If you have additional questions, feel free to contact me.
the Telerik team