Sorry, but this is absolutely untrue. The Ribbon UI introduced in Office 2007 was a massive change functionally and visually. You went from a static toolbar that would just show and hide buttons to live categories which not only resize but change their options and layout as you customize or resize the window. There's now drop downs, input fields built in, live previews in the document as you hover over tools and options, and more.

Same for the new Backstage UI introduced in Office 2013 for saving files, viewing recents, and other file and option operations. You have full screen animations and interactions.

Hell, Microsoft even made the text cursor fade in and out instead of blinking, which needs more processing power.

Could Microsoft have optimized it more? Yes. But they definitely have added tons to it since the 90s and even mid-00's to justify why it's slower.