When I press alt+tab the window that comes up highlights the currently selected window. As I press alt+tab the transition to the next window is animated (the highlight drifts to the next one).

How can I disable that animation and have it immediately highlight the next one?

I've already disabled all desktop effects, but it doesn't appear to change this.

Open System Settings > Window Behaviour > Task Switcher. Untick "Show outline of selected window", and set "Effect" to "No Effect". You may also wish to untick "Display list while switching"

While this does let me customise more to my liking there is still no way to turn off the slow sliding animation (there effect was already set to "No Effect").

The informative header helps though, as it switches immediately (no transition effect).

You may wish to file a bug report at bugs.kde.org. Please include information such as distribution, X server version, and graphics driver in use.