And for some reason, unzipping Eclipse was being incredibly slow. Like, 300kbps slow. Which is about 100 times slower than it should be (SSD, i5 processor). So I checked what the resource monitor, to see if something was hogging the disk, or using all of the CPU.
And found that Windows Defender was pegged solid at 25% of CPU. Which, as I have four cores, meant that it was using all of one of them. This was annoying for two reasons: (1) 100% of a CPU and it still couldn't manage more than 300kB/s of scanning? (2) Only using one CPU? What is this, the 20th century?
As it happens, I have a Norton license with 3 devices available on it. And Norton underwent a major rewrite a couple of years ago which meant that it was actually pretty efficient (and easy to install/uninstall, which used to be hell). So I logged onto their site, grabbed a licensed copy, and installed it. Which took about three minutes in total, and worked seamlessly.
I then ran a quick scan (which removed 29 tracking cookies), restarted my laptop (to be on the safe side), and unzipped Eclipse again. Which got me about 30MB/s, while using almost no CPU at all.
I normally recommend Microsoft's own anti-virus to people, on the grounds that it comes with the OS, and seems to score reasonably well on effectiveness. I think I'll have to stop doing that (or at least offering caveats).
Original post on Dreamwidth - there are comments there.