Re: Still asking why (anything post XP)
Windows just gets slower over time, the more modern incarnations suffer more through increased complexity. While I haven't wasted a mammoth amount of investigation time into it, there are a few main culprits:
1) The Registry. Bloats and bloats and bloats, never shrinking, always getting slower.
2) The entire .COM / ActiveX DLL hell, requiring huge numbers of the same libraries, in a version number hell, all underpinned by the registry. The smallest of changes adds even more bloat to the registry.
3) .NET - it is neither fast nor elegant. The more this becomes used for operational parts of an OS, the slower the OS will run. When .NET is used with device drivers, it gets worse - luckily this is still very rare.
4) Anti-Virus and Anti-Malware software. While these are more stable than they used to be, they do seriously impact system performance. With more templates and variations to check with every new virus / malware that is released, the more work these systems need to do.
5) Application update software. Little can bring a system to its needs quicker (ha) than multiple competing applications all running their own update check process every time the system starts. A good, flexible API and service from Microsoft could have helped with this, but no... and the hoops that some of these applications go through to provide background updates without a stream of UAC prompts is just horrible. And then the AV/AM software checks every file access and update by each of these update processes....