In this video, Chris Bryant of Microsoft makes an offhand comment that the components of the new Microsoft Office 2010 for Web are “some of the biggest Javascript applications ever built”.

The Javascript performance in Internet Explorer generally lags behind that of its competitors (Firefox, Safari, Chrome) — though the importance to the end user experience is debatable. IE has made strides, but the others have improved performance by orders of magnitude.

The reason is simple: Javascript has never been really strategic for Microsoft. They obviously support it and improve it, but it’s not a differentiator in any way. It’s just a matter of being “good enough”.

Other languages, such as C# and Visual Basic, have obvious value for Microsoft. They are the main languages that are used by its developer ecosystem, driving Windows sales, and MS makes money selling development tools like Visual Studio. Javascript, however, has never fit into Microsoft’s finances.

But now that MS has committed to a rich, cross-browser version of Office, Javascript becomes strategic; it now underpins something that they sell.

This is the first time that one of Microsoft’s cash cows truly depends on the browser, and therefore browser performance. If they are competitively forced to sell a Web version of Office, you can bet that they want people to use IE.

My prediction: an IE 8.5 (beta/preview/whetev) will be released within 6 months of Office 2010’s ship date. The major bragging point will be a (let’s say) 4x speedup in Javascript.

Microsoft is generally very good at squeezing performance out of very complicated software — if they have the incentive. Now they do.

Related: Why not Silverlight?