Visual Studio 2010 - XAML Editor Extraordinarily Slow

In Visual Studio 2012:

Go to

Tools > Options > TextEditor > XAML > Misc 

Check Always open documents in XAML view and uncheck Automatically populate toolbox items.

In Visual Studio 2015, 2017:

Go to

Tools > Options > XAML Designer 

uncheck Automatically populate toolbox items.

This fixed my same problem.

If you don't need the visual designer, you can select a different editor in visual studio:

Right click the xaml file -> Open With... -> Source Code (Text) Editor

You will only lose the split view, intellisense etc. should still work.
If you need to check something in the designer you can still click 'view designer' to open the normal XAML editor again.

If you want to improve the performance of the visual designer, try checking DesignerProperties.GetIsInDesignMode in your code-behind. The visualiser instantiates your controls to know how to display them, thus executing parts of the code-behind.

Like the OP, I had extreme lag in the xaml editor on a project that relied heavily on DevExpress WPF controls. After trying unsuccessfully to resolve this issue with the other solutions posted here, I eventually tried deleting my Solution User Options (.suo) file, which is usually located in the same folder as the solution (.sln) file. This immediately resolved the issue. You may want to read this post about what visual studio stores in the suo file so you can reset anything important (such as build configuration - mine always defaults the active solution platform back to "Mixed Platforms" for example)