Odd XP Shutdown Issue

[#1]

This isn't a problem as such - more a mystery.
Up until about a week ago, when I shut down my system (XP Pro SP3) it used to take about 10-15 seconds before it would power off.
Since about last Friday, it only takes about 4 seconds.
I didn't defrag or run a full AV scan, Spybot or Malwarebytes before the change (and haven't since), and the only thing which I can see might be different is that I installed updated drivers for my Epson printer, but I'm pretty sure that the quicker shutdown started before I did that.
As I say, I'm not bothered by it, only curious.
Any thoughts?

[#2]

Computers tend to take longer to shut down than they used to.  I presume this is because features like System Restore make a record of system files and settings.  Could this have been turned off? 

[#3]

I find there are wide variations, especially when there have been updates of any type. These can take one or two reboot cycles to stabilise.
The biggest delays seem to come from networking or peripheral "issues"; I've seen PC's that shut down in a few seconds normally but if a certain peripheral or network is connected can take minutes; presumably some process timing out waiting for something it never gets.