Here’s my steps and workarounds from the very beginning (in Visual Studio 2012), hopefully some part of this will help fix whatever error you are encountering.

File -> New Project
.NET Framework 4.5
Visual C# -> Web
ASP.NET Web Forms Application
Add AjaxControlToolkit version 7.0123 (dll is actually 4.5.7.123 which is January 2013 I believe) via NuGet

Open Default.aspx
Add a calendar to the BodyContent / MainContent:

<asp:TextBox ID="myTextBox" runat="server" />
<ajaxToolkit:CalendarExtender ID="myCalendar" runat="server" TargetControlID="myTextBox" Format="dd/MM/yyyy" />

Run it up (F5 will do)

It might error about ASP.NET Ajax 4.0 scripts:
0x800a139e – JavaScript runtime error: AjaxControlToolkit requires ASP.NET Ajax 4.0 scripts. Ensure the correct version of the scripts are referenced. If you are using an ASP.NET ScriptManager, switch to the ToolkitScriptManager in AjaxControlToolkit.dll.

I’ve also seen it error about:
‘MsAjaxBundle’ is not a valid script name. The name must end in ‘.js’.

No bother, let’s remove that reference from the Site.Master, so:

<asp:ScriptManager runat="server">
    <Scripts>
        <%--Framework Scripts--%>
        <asp:ScriptReference Name="MsAjaxBundle" />
        <asp:ScriptReference Name="jquery" />
        <asp:ScriptReference Name="jquery.ui.combined" />
        <asp:ScriptReference Name="WebForms.js" Assembly="System.Web" Path="~/Scripts/WebForms/WebForms.js" />
        <asp:ScriptReference Name="WebUIValidation.js" Assembly="System.Web" Path="~/Scripts/WebForms/WebUIValidation.js" />
        <asp:ScriptReference Name="MenuStandards.js" Assembly="System.Web" Path="~/Scripts/WebForms/MenuStandards.js" />
        <asp:ScriptReference Name="GridView.js" Assembly="System.Web" Path="~/Scripts/WebForms/GridView.js" />
        <asp:ScriptReference Name="DetailsView.js" Assembly="System.Web" Path="~/Scripts/WebForms/DetailsView.js" />
        <asp:ScriptReference Name="TreeView.js" Assembly="System.Web" Path="~/Scripts/WebForms/TreeView.js" />
        <asp:ScriptReference Name="WebParts.js" Assembly="System.Web" Path="~/Scripts/WebForms/WebParts.js" />
        <asp:ScriptReference Name="Focus.js" Assembly="System.Web" Path="~/Scripts/WebForms/Focus.js" />
        <asp:ScriptReference Name="WebFormsBundle" />
        <%--Site Scripts--%>

    </Scripts>
</asp:ScriptManager>

Now becomes:

<asp:ScriptManager runat="server">
    <Scripts>
        <asp:ScriptReference Name="jquery" />
        <asp:ScriptReference Name="jquery.ui.combined" />
        <asp:ScriptReference Name="WebForms.js" Assembly="System.Web" Path="~/Scripts/WebForms/WebForms.js" />
        <asp:ScriptReference Name="WebUIValidation.js" Assembly="System.Web" Path="~/Scripts/WebForms/WebUIValidation.js" />
        <asp:ScriptReference Name="MenuStandards.js" Assembly="System.Web" Path="~/Scripts/WebForms/MenuStandards.js" />
        <asp:ScriptReference Name="GridView.js" Assembly="System.Web" Path="~/Scripts/WebForms/GridView.js" />
        <asp:ScriptReference Name="DetailsView.js" Assembly="System.Web" Path="~/Scripts/WebForms/DetailsView.js" />
        <asp:ScriptReference Name="TreeView.js" Assembly="System.Web" Path="~/Scripts/WebForms/TreeView.js" />
        <asp:ScriptReference Name="WebParts.js" Assembly="System.Web" Path="~/Scripts/WebForms/WebParts.js" />
        <asp:ScriptReference Name="Focus.js" Assembly="System.Web" Path="~/Scripts/WebForms/Focus.js" />
        <asp:ScriptReference Name="WebFormsBundle" />
    </Scripts>
</asp:ScriptManager>

Next is ToolkitScriptManager, the snippet above now becomes:

<ajaxToolkit:ToolkitScriptManager runat="server">
    <Scripts>
        <asp:ScriptReference Name="jquery" />
        <asp:ScriptReference Name="jquery.ui.combined" />
        <asp:ScriptReference Name="WebForms.js" Assembly="System.Web" Path="~/Scripts/WebForms/WebForms.js" />
        <asp:ScriptReference Name="WebUIValidation.js" Assembly="System.Web" Path="~/Scripts/WebForms/WebUIValidation.js" />
        <asp:ScriptReference Name="MenuStandards.js" Assembly="System.Web" Path="~/Scripts/WebForms/MenuStandards.js" />
        <asp:ScriptReference Name="GridView.js" Assembly="System.Web" Path="~/Scripts/WebForms/GridView.js" />
        <asp:ScriptReference Name="DetailsView.js" Assembly="System.Web" Path="~/Scripts/WebForms/DetailsView.js" />
        <asp:ScriptReference Name="TreeView.js" Assembly="System.Web" Path="~/Scripts/WebForms/TreeView.js" />
        <asp:ScriptReference Name="WebParts.js" Assembly="System.Web" Path="~/Scripts/WebForms/WebParts.js" />
        <asp:ScriptReference Name="Focus.js" Assembly="System.Web" Path="~/Scripts/WebForms/Focus.js" />
        <asp:ScriptReference Name="WebFormsBundle" />
    </Scripts>
</ajaxToolkit:ToolkitScriptManager>

But of course this fails with:
Could not load file or assembly ‘System.Web’ or one of its dependencies. The system cannot find the file specified.

Remove the Assembly=”System.Web” part from the ScriptReference so we have have:

<ajaxToolkit:ToolkitScriptManager runat="server">
    <Scripts>
        <asp:ScriptReference Name="jquery" />
        <asp:ScriptReference Name="jquery.ui.combined" />
        <asp:ScriptReference Name="WebForms.js" Path="~/Scripts/WebForms/WebForms.js" />
        <asp:ScriptReference Name="WebUIValidation.js" Path="~/Scripts/WebForms/WebUIValidation.js" />
        <asp:ScriptReference Name="MenuStandards.js" Path="~/Scripts/WebForms/MenuStandards.js" />
        <asp:ScriptReference Name="GridView.js" Path="~/Scripts/WebForms/GridView.js" />
        <asp:ScriptReference Name="DetailsView.js" Path="~/Scripts/WebForms/DetailsView.js" />
        <asp:ScriptReference Name="TreeView.js" Path="~/Scripts/WebForms/TreeView.js" />
        <asp:ScriptReference Name="WebParts.js" Path="~/Scripts/WebForms/WebParts.js" />
        <asp:ScriptReference Name="Focus.js" Path="~/Scripts/WebForms/Focus.js" />
        <asp:ScriptReference Name="WebFormsBundle" />
    </Scripts>
</ajaxToolkit:ToolkitScriptManager>

It seems the new web forms project template adds ‘Microsoft.AspNet.ScriptManager.MSAjax 4.5.6′ package, this appears to conflict with the toolkit, so remove this via “Manage NuGet Packages”
Visual Studio might still leave the dll in your bin directory even after a clean, make sure you manually clean that out.

In this article I will sharing few tips to improve the performance in ASP.NET site. This is the things that I have done:

1. HTTP Compression

HTTP compression is used to compress page content from the server end. It compress HTTP requests and responses, so is a great performance improvement. My project is hosted in Windows Server 2003, and I implemented HTTP compression after reading this article.

2. Disable Possible ViewState

View State allows the page state to be persisted with the client, and it does not require cookies or server memory. View State saves data in a hidden input filed inside a page. Definitely, it is a powerful feature, but the drawback is it increases page size and memory allocation in the server.

So, we should avoid View State where it is not necessary. Especially, in the case of DataGrid controls, we should avoid View State as it loads all the grid data in the page state.

In my application, I disabled View State for most of the controls and pages where View State is not necessary. This made the page sizes smaller. 

3. Changes in the Web.Config File

a. Use page caching:

This will save your page only for a certain amount of time, and the page will be faster to load. But, remember that if your page data changes frequently, it is not a good idea to use page caching.

<caching>
<outputCacheSettings>
    <outputCacheProfiles>
        <add name="cached" duration="600"
            varyByParam="none" enabled="true"/>
    </outputCacheProfiles>
</outputCacheSettings>
</caching>

b.Remove unnecessary httpModules from the web.config:

<add name="ScriptModule"
     type="System.Web.Handlers.ScriptModule, System.Web.Extensions,
           Version=3.5.0.0, Culture=neutral,
           PublicKeyToken=31BF3856AD364E35"/>
<remove name="WindowsAuthentication" />
<remove name="PassportAuthentication" />
<remove name="AnonymousIdentification" />
<remove name="UrlAuthorization" />
<remove name="FileAuthorization" />

c.Turn off trace:

<trace enabled="false" pageOutput="false" />

d.As I have used membership, I disabled automatic save for profiles:

<profile enabled="true" automaticSaveEnabled="false" />

e.Set debug=false:

<compilation debug="false">

4. Optimize Stylesheets

It is important to clean up style sheets and remove unnecessary code from style sheets because it increases the load time. Remove all unnecessary classes from style sheets and try to use a single CSS file.

5. Optimize JavaScript

6. JS and CSS File Position

According to the advice from some good articles, I put the CSS file declaration at the top of my master page. It is recommended to call CSS files at the top of a page because the page rendering will progressively become efficient.

I put JavaScript file links at the bottom of the main master file. If we put scripts at the top of the page, then it will cause unnecessarily high load times.

7. Client-side Script for Validation

For page validation, I use client-side validations instead of postbacks. This way, I reduce round trips in pages.

The above are few trips to improve your ASP.NET performance.

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