Interestingly, as I understand it, the Office-for-Mac programs weren't developed for Windows and then ported to OSX, so that wouldn't explain the difference in performance. For instance, Excel for Mac preceeded Excel for Windows. And the first Word for Mac was a different program from the first Word for Windows, with (at least according to Wikipedia) a WYSIWYG interface the Windows version lacked. I.e., I think that, in general, Office for Mac was developed directly for the Mac.
The history you're talking about happened in the mid-1980s. The computing world looked very different then, and not much from that time is still relevant today. In fact, it only took about 10 years for things to change in a huge way.
In 1985, when Excel 1.0 was released as a Mac-only program, Windows was horrible me-too junk that nobody used. When they released the first PC version of Excel in 1987, Microsoft sold it primarily as a DOS program. It was actually a Windows program, bundled with an Excel-only Windows shell. You'd run EXCEL.EXE from the DOS command prompt and that would start the bundled copy of Windows just to run Excel, then it would quit out of Windows when you were done using Excel. This bundling was necessary because the audience interested in Excel on PCs was far larger than the audience interested in paying for full standalone Windows just to run Excel -- that's just how relevant Windows was in 1987.
Just 10 years later, in 1997, Windows had won, Win95 was an incredible success, and Microsoft was a true monopolist. After years of overpriced blah products and failed (but very expensive) attempts to replace the rapidly aging technology at the core of MacOS, Apple was close to bankruptcy, and had just made a desperation play to acquire NeXT, which came with an infamous executive named Steve who had yet to lose the stink of his previous departure from Apple. Nobody knew if that acquisition would actually work out, and in fact the smart money was on it not doing so.
By that timeframe, it wasn't true that the Mac versions of Excel and Word were the reference and the Windows versions the copies. Microsoft was putting far more resources into the Windows versions, and the Mac versions needed to have stuff backported to keep up.
In fact, one of the key events in Jobs' return to Apple concerned the future of Mac Office. Rumors were swirling that Microsoft was going to drop it, and why wouldn't they? Macs were less than 5% of the personal computer market, it couldn't be very profitable for Microsoft. Then Apple won a lawsuit against Microsoft based on QuickTime patents. In the settlement negotiations, Jobs got two concessions out of Microsoft: one, Microsoft purchased a big chunk of nonvoting Apple stock and made a public statement expressing confidence in the future of the company, and two, Microsoft committed to several years of providing Mac Office (and maybe Internet Explorer too? I don't remember clearly).
These were huge deals at the time. Apple was in serious financial trouble and the stock purchase raised some cash, and the assurance that Mac Office wasn't going away soon put to rest one of the most obvious death-spiral scenarios for the Mac.
Getting back to the main topic - Microsoft absolutely had more resources behind the Windows versions at that time, and had been doing that for many years. Mac Office was poorly maintained, rarely at feature parity with Windows and never at performance or stability parity. (This was all part of why everyone feared that Mac Office was just going to be dropped altogether.)
. Actually, I did include a result with Pages. Not surprisingly (both since since it is written by Apple for Apple, and since it's a lighter program with much less functionality than Word), it was quite a bit faster.
