The issue started in the Dolphin side panel a few releases back, but it wasn't too bothersome to me because I don't use the places at the bottom often.

But the same issue is now affecting Application Launcher. My 'Favorites' contains only about 2.5 items more than the height of the launcher would allow, but scrolling down that takes more than two wheel scrolls. This is also true for all the other items of the launcher (such as Applications, Computer). Holding the scroll bar by left-clicking and dragging is also affected (the mouse has to travel a longer distance to scroll the same distance in a non-affected frame, i.e. the scroll bar in Firefox for example keeps up with the mouse pointer, and moves at the same speed, whereas it lags and the pointer passes the bar in Launcher and the Dolphin side panel).

Scrolling sideways in the Dolphin side panel, and in all directions in the 'content' area (where files and directories appear) is in normal speed.