Plasma 5.10: popups freeze the entire application for half a minute; GTK apps also buggy

I am admittedly using Antergos (arch with graphical installer and some peasant features). Over in r/antergos we could not find a solution (or even what the problem is). Maybe you can help?

When a popup is opened, the window contents are not drawn for a while (30 secs roughly). Then everything works. By popup I mean for example the "really overwrite this file?" or "settings have changed, apply them?" windows. This happens in KDE applications like dolphin, okular, system settings.

GTK applications also act weird: meld and nautilus are just black boxes for some time, and then they work but really slow. Meld has a titlebar always (also when black), but nautilus just is a movable black rectangle.

I have this problem on 3 different machines all running basically intel hardware, no dedicated graphics. If there is anything helpful I could post please let me know.