To be fair, a lot of that has to do with the fact that modern machines are faster than most things they are used for. When the machine already runs your software at full speed, there isn't much else to get out of it. For example, I have a first gen Atom with Ion video that I use for XBMC. In another room I have an i5 for the same purpose. They both run XBMC at full speed, so even though the i5 is WAY faster, I only get a small improvement in usability.
Yes and no. It's true that CPU speed is kind of approaching a saturation point (at least for everyday tasks, not so much for heavy-duty stuff like audio/video editing,) and while the pointless glitz that gets shoehorned into commercial OSes bogs things down, it's easy enough to turn off (one of my first steps with any new install.) But that's not the only factor for performance, or even the most important. You know what is? Memory. If you have 2+GB of RAM on an XP machine, you can turn off virtual memory and enjoy sweet, sweet freedom from disk-banging when you have more running at once than a web browser and Notepad. Vista? 2GB is the
basic minimum for reasonable operation. Hard drives keep getting roomier, but Vista jumped from XP's requirement of 1-2GB for a typical install to a minimum of
15GB free disk space. It's not so vast a difference now that 1TB drives are fairly easy to come by, but it's still appalling.