The animation speed settings is under Display and Monitor -> Compositor (kwincompositing kcm) which makes it hard to find. As it affects desktop effects, it should be near them / next to them (Desktop Behaviour -> Desktop effects) . When looking for it I always look through all the items in the Workspace group, sometimes also Appearance, only remembering after some time that it was actually grouped with the other compositor configuration which got moved to Display... I don't think that new / nontechnical users would connect that setting with Display / Compositor, but instead would expect it next to the desktop effects / workspace settings, maybe window behaviour... Reproducible: Always
Forgot to add that searching for "animation speed" in systemsettings actually highlights "Desktop Behaviour" which is clearly wrong right now.
Search for 'animation speed' text - shows nothing, search for 'animation' shows 'Window Behaviour' entry Plasma: 5.12.4 Apps: 17.12.4 Frameworks: 5.44.0 Qt: 5.10.1 Kernel: 4.14.32-1-MANJARO OS: Netrunner Rolling
It is in 5.18