This has to be taken in the context of the time. This is the era of Apache on a low end Pentium being able to survive slashdotting whilst IIS on NT 4 would fall over on much grander hardware.
Arguably much of the difference can be attributed to being able to get Windows running easily - forgoing the painful process of having to learn what you’re doing (and at the same time learning how to tune it and where the slowdowns might be).
We’re now in a much different world. (Arguably the main difference remaining is that a Linux comes with “everything” whereas the others need out-of-band installations: in the 90s those things cost money whereas they were free on Linux - compilers used to be a pay product!)
I'm not arguing for the superiority of Windows, I'm pretty sure Linux+Apache was superior back then to Windows+IIS and is superior now. However, you could probably play audio, open a Word document and play 3D accelerated games on said NT box - not necessarily things you'd want to do, but the complexity to support these use cases was there anyway.
My point is, full-fat desktop OS-es are much more complex beasts that a bare-bones Linux server install, that probably is a bunch of init-scripts and a server process on top of the kernel. Once you try to turn a UNIX into a desktop OS, it'll have the same problems as Windows, for the same reasons.
Arguably much of the difference can be attributed to being able to get Windows running easily - forgoing the painful process of having to learn what you’re doing (and at the same time learning how to tune it and where the slowdowns might be).
We’re now in a much different world. (Arguably the main difference remaining is that a Linux comes with “everything” whereas the others need out-of-band installations: in the 90s those things cost money whereas they were free on Linux - compilers used to be a pay product!)