It sounds like you're saying that the higher performance I see for the Windows (vs Mac) version of MS Office could be due to various platform-dependent characteristics that are external to MS Office itself—such as the relative performance (or suitability) of Windows vs. Mac system libraries, and other characteristics of the platform that in turn affect the performance of the app.
To an extent. Part of it is that Windows and macOS have different philosophies for development. I’ve always likened Microsoft APIs to Lego blocks. You get everything you need to do the task, but some assembly is required. Apple on the other hand thinks about scenarios, but wants to funnel you into their libraries and flows for the most benefit. So Apple APIs have a
tendency to be higher level than their equivalents on Windows. You can get really good perf from Apple’s libraries with less work, but it tends to make cross-platform development more complex if libraries don’t exist at the same level on Windows. It can be akin to OpenGL vs Metal/DirectX. An app that has a lot of its own scaffolding to account for lower level APIs doesn't always mesh well with higher level APIs. This mismatch I believe is a reason why Apple went hard and fast with Metal as a replacement for OpenGL and OpenCL. It brought those APIs to where developers were moving to on other platforms, and enabled better performance as a result.
What I'm about to say doesn't necessarily apply to Office. Again, this is a hugely complicated suite with a lot of interacting parts. Measuring and attacking performance issues on software this big is a complex effort and every time I've done it, I've learned something new and weird about Apple's platform. However, I want to give an example of where differences can apply when it comes to rendering text (something hugely important to an app like Word or Pages).
DirectWrite has some rather useful aspects to it that help in situations that allow for efficient drawing of glyphs to a D2D surface. CoreText serves a similar role on Apple platforms, but is tied to CoreGraphics rather than say, Metal or CoreAnimation which is where GPU acceleration comes into play. CoreGraphics, while it does play nice with hardware acceleration for compositing, is not quite as nice as a fully accelerated D2D surface when it comes to text last time I worked this low down. I honestly don’t know if Apple’s done anything in this space recently. What I do know is that UIKit on iOS is generally more modern when it comes to hardware acceleration than AppKit is on macOS, which is a little disappointing. But generally I want to avoid drawing everything all the time, so this mostly impacts things like scrolling framerate when we need to redraw content.
But it also impacts pagination. Say I want to understand how large a run of text is on a page, I need to know the font metrics for the font(s) used. So I need to ask someone about it. I can either ask my own library that has to be able to read font data itself, or I can ask the system to do it for me. DirectWrite and CoreText both have good APIs for this, so it makes the most sense to use the system APIs, IMO. But it means any differences in performance between the two ripple throughout my layout code. And since pagination time is
roughly linear for the length of the text to paginate, that OS call can wind up impacting your tight loop. But is it worth going to my own library if the platform isn't fast enough? It depends... a lot. Font work is a whole regime onto itself, so at some point, I'd argue you
want to let DirectWrite/CoreText own it, even if it means accepting some drawbacks. To a point.
I was focused on the extent to which MS had better optimized Office to work with Windows, but what you're saying is about the other half, namely that Windows is better optimized (than MacOS) to work with Office. It makes sense that MS co-developed Office and Windows to run as well as possible with each other, and that this synergy is better developed than that between MacOS and Office. Office is probably the single most commercially important aftermarket program for the Mac (when it comes to the Mac's product share). Thus I assume there is extrensive communication between MS's Office team and Apple, such that, if the MS Office team told Apple that they could get much better performance if they did "X" to Mac OS, that's something the MacOS developers would seriously consider. But the co-development surely isn't at the level it is within MS between Windows and Office.
Yes, there are some things about Windows that likely makes it better suited to how Office interacts with the platform. However, those things could very well be true for say, Photoshop. Or really any large monolithic app that sits on low level APIs because that's all that existed when they were written initially. I'd honestly half expect Office and Photoshop are both better suited for pre-OS X Apple than post-OS X Apple. Moving away from Carbon forces developers to adopt higher level APIs than what they were using before (and still use on Windows). UIKit even more so: just look at keyboard/text input on iOS vs AppKit. I'd argue this is a hurdle that is effectively true for the whole legacy app space.
The one thing I’ve learned about Apple that surprised me is how little they like to play favorites with 3rd party devs. Do they pay attention? Yes. Are they willing to help developers make good products if it’s considered important? Absolutely. Is Apple likely to make a change that mostly enables one developer, or at a single developer's behest? No.
Apple only deals to themselves.