slow performance rendering table with data reader from code behind in C#, is there a better way?

I have a function that pulls info from a database using a datareader. All the info is placed into a single string using several appends. The string is then output into a panel.  The string ends up as html and is used to render a table. While this function works, it is very slow. Is there a better way to do this rather than loading it all into a single string. I thought about response.write but I've read thathat you shouldnt do that in ASP.net. here is my code:
public void GridViewDefaults()
{

string strSQL = "SELECT GAME, EXACT, EXACTSKIP FROM Combined_History WHERE GAME > " + TextBox1.Te ...