DataGridView selection performance

Question

I have the following environment: - DGV in a virtual mode - 20 columns and 300,000 rows - Selection mode is CellSelect

I'm selecting small regions of cells (on the visible area) using the mouse and get a big delay (the selection is repainted only a second after I moved the mouse).

Now the tricky part: I scroll to the very bottom of the grid select cells again and there no such extreme delay! So basically this delay gradually increases the closer you get to the top (looks like it grows linearly).

I can't provide my code "as is" as it's a part of the larger application. I understand that it is hard to check this without any code and I don't expect a ready solution. But I hope to get some ideas that would help me understand why the scroll position affects the selection painting delay.

Thanks