Slow on Text Validation on Grids with multiple column and rows

I've managed to do validation for grid when it sets EditMode (BTW ... we are using RadGrid from Telerik) by modifying GridBoundColumn control below. What I found is that it was ok it's grid has no many rows and columns but once you have many rows/columns and it's slow jumping between tabs (I have a multiple page inside tab and one of them is displaying this grid). This this grid like an excell with each cell has textbox control on it and the size is varies depending on the situation.

Any ideas to improve this? Or do I need to take different approach for doing this?

Thank you



 public class dcGrid ...