Lo Wang,
You make some valid points, and I agree with them, in part.
For a Pentium4, I would probably not go beyond XP myself. Then again, if I care about performance in general, I will not use a Pentium4 as my primary machine. It would feel a bit strange to talk about performance advantages of a certain OS, which I doubt can ever go beyond 20% among the generations we're looking at, and at the same time use outdated hardware that's almost an order of magnitude slower than modern systems. But since this thread was specifically started to talk about alleged performance advantages on older hardware, I do agree that your point has merit.
Regarding tweaking XP vs NT6 - I will have to take your word on it. Many years ago I was also in the habit of tweaking the OS, but I stopped at some point, because, frankly, I saw no returns significant enough (if at all) to justify my time spent. Even if as you say, once you learned to do it, it only takes a few minutes to do, there is still the maintenance, and making sure that you "use the right applications", as you put it, and that nothing (like an update or a third-party app) "accidentally" changed your settings, etc. At some point I stopped worrying about it, and just started using my systems and enjoying them. So, come Vista, I never bothered with tweaks, except some very basic ones (like, well, disabling Windows Search&Indexing service which I never use anyway).
So all in all, I can't present exact data with what's tweakable and what's not, and what positive/negative side effects each of them has. I have seen extensive guides on such tweaks for every Microsoft OS, and claims of amazing improvements, but given my own past experience with some of them (again, mostly pre-Vista and nothing past-Vista), I take them with a grain of salt, and don't bother with them.
However, I will be happy to learn more of your experience, like which tweaks you use on XP, and what you tried on NT6, and where you "hit the wall" with it so to speak. If you think this too off-topic for this thread (even though, IMO, it's quite on-topic), then perhaps you have shared this info in prior discussions and can point me to it.
Regarding privacy/security issues, I agree with you that this is not the right thread, although IMO, they are also often blown out of proportion and poorly understood (in the sense that people believe something is a violation, while it is quite harmless).
A final point about UI - I got use to the XP UI myself over the years, and found it adequate. Vista did not really add anything significant to me (Sidebar is potentially cool, but is not my cup of tea). I do like some of the things introduced in Win7 and Win8 (like jump lists, improved Aero Peek, Aero Snap, Win+X in Win8, better integrated handling of multiple monitors, and probably a few others I can't recall at the moment). It is clearly a case of "you can go without it indefinitely, but once you try it, you will miss it if you go back". Yes, you can probably add most of it through third-party tools, but I'd rather just use one third-party tool to customize the start menu on Win8, then a plethora of utilities to snap extras on XP.
