changing visibility contentplaceholder asp.net not working immediately

[tyncseectx5n4]

I have a simple asp.net webpage that contains of 2 \[code\]RadioButton\[/code\] lists. When the form is submitted some more complex code is executed, which takes some time (5-10 seconds) to execute.I want to simply change the visibility between 2 \[code\]ContentPlaceHolder\[/code\] before executing that code. The switching works but only after the complex code is executed. The result is that the page freezes and changes after some time.My aspx code:\[code\]<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent"><asplaceHolder ID="requestpage" runat="server" Visible="true"> <p> </p> <h1> Navigatie update</h1> <p> </p> <div style="text-align: left; width: 300px"> <p> Kies een site</p> </div> <p> <asp:RadioButtonList ID="pubtargetid" runat="server"> </asp:RadioButtonList> </p> <br /> <br /> <div style="text-align: left; width: 300px"> <p> Kies een taal</p> </div> <p> <asp:RadioButtonList ID="chosenLanguage" runat="server"> </asp:RadioButtonList> </p> <p style="color: red"> <asp:Label ID="errorMessage" runat="server"> </asp:Label></p> <p> <asp:Button Text="Start update" OnClick="Submit" runat="server" /> </p></asplaceHolder><asplaceHolder ID="responsepage" runat="server" Visible="False"> <p> </p> <h1> Navigatie update</h1> <p> Navigatie wordt ge