The download progress notification duplicates information found in the browser and system tray, so there is no need for it to be shown for the entire duration of the download. So the same info is repeated in three places and that's really unnecessary. Two seconds should be enough and then it should close itself, the user can always monitor detailed progress by clicking on the icon in the systray. Of course the notification should still appear on download completion.
In System Settings → Notifications uncheck "Application progress: Keep open during progress"
Ah, I forgot there is a toggle for everything on KDE. thanks! Will you consider changing the default though? I don't think the current behavior makes sense and it's not something the user should have to configure.
This popup is also used for copying files in Dolphin, which doesn't have its own job tracking.
I think the tray is sufficient for that. It still shows up in the tray. Notifications are in general very well done, but the main problem is that they are hard to get rid of. There is no dismiss shortcut and many of them don't close even after you act on them. For example Chrome email notifications stick around even after you click the link in the notification. This issue is just one example of this larger problem. It may seem trivial but, repeated it dozens of times per day and it really does get in the way of the stuff you're actually working on and the desktop should never do that.
Again, nothing wrong with showing the notification but I don't see any reason for it to ever persist for the duration of a process by default. Plasma makes it really to check previous notifications now anyway.
*really easy.