I've not known, Explorer to open instantly, or what I would call instantly, even on a fresh install, for some time IIRC. Heck, the amount of times I hit win+R to kick something off and that takes so long to open that the first couple of characters are missed and go to whatever window I had in focus at the time, is irritating. It certainly gets a lot worse when Explorer has been running a while (presumably this is why it has its own special case in Task Manager – pick Explorer and “end task” becomes “restart”) and its memory use has grown significantly (this is likely at least in part due to shell extension though the only one with any complexity I have installed here is MS's own TFS integration, and I have nothing like that running at home ATM).

Other things similarly take a while, like registering an update made elsewhere. These can be quite arbitrary, sometimes pretty instant, sometimes taking many seconds, and sometimes not updating at all until I explicitly refresh. I've even copied a small file to the same directory and waited several seconds before it showed up.

> Did you try to reinstall Windows?

For no sane desktop/similar operating system should “completely reinstall” be an off-hand answer for any but a massively significant error…