Try going to the "View" tab in the Ribbon and switching to the "Draft" layout. Someone mentioned in the comments of the following blog post that you could type as fast as you can in this view without lag:
http://blogs.msdn.com/b/murrays/archive/2008/02/17/hidden-math-features-in-word-2007.aspx
Hello everyone,check out my free shorthand notation for Office 2010 if want to enter math very quickly and easily using the native math facility of Word 2010, PowerPoint 2010 or OneNote 2010. The documentation of my notation also documents almost everything that is available in the math facility...