Just wanted to start collecting lesser known causes for Windows sluggishness.
Depending on how big the local network is, what folders, drives and devices you have "explored" and what add-ons you've installed, Windows Explorer open can chip away at your computer's speed when left running. I suspect that it's scanning the system to make it run faster (ironically) but can result in disk activity making the computer sluggish when you in the middle of something. It's not uncommon for me to have several instances of Windows Explorer open when moving folders and files around but now I close them when I'm done. To minimize the number of clicks to get at certain groups of files quickly (which I used to do by just keeping Windows Explorer instances open) I added short cuts to the important folders on the Windows desktop. This kind of sluggishness is a lot more noticeable on my older Windows Vista computer.
Flash On Web Pages
Flash components on open web pages (even on tabs that are not visible) can slow down your computer. The makers of Flash improved this significantly in later years but there are still tons of older components out there on web pages that can really tap out your computer. A quick test to see if this is the cause of sluggishness is to just shut off the browser (assuming that you weren't experiencing the sluggishness while browsing). It can take tens of seconds for the browser to completely close but if the computer is still slow with browser closed you at least know that something else is contributing. You can also just close tabs that you suspect are a problem. You can always get back using the browser history if the problem didn't go away.