The first time the Overview effect is activated after KWin starts or restarts, it will not appear. The second time it is activated, it will appear. This reproduces for me on both X11 and Wayland.
The lag can be caused by the static plugin issue. It takes a lot of time to load all images.
Isn't that a duplicate of https://bugs.kde.org/show_bug.cgi?id=445132?
I don't think so. I don't experience that issue, and also lag on first opening is different from not opening at all the first time it's invoked.
Fixed with the latest commits on git master.