> Yet I can't identify anything that W10 does better (for me) than W2000 that justifies its sluggishness on a C2D.

Core 2 Duo was introduced 15 years ago (2006). Almost a decade before Windows 10 was released.

Windows 10, and all modern operating systems, are designed around the availability of modern GPUs. If you try running it on an ancient machine without modern graphics acceleration and without enough CPU power to handle it in software, it's going to feel sluggish.

Windows 10 is perfectly fine and snappy on every machine I've used it on in recent history.