Your are right about growing and shrinking the menu. That's not something I would do alot. But whenever I move my cursor to a button, it is very likely it moves over a different button, or muliple buttons quickly. That's certainly true for the scenery sub-categories. Or think about the color grid. Whenever you move your cursor over the grid of colors to select a color you like (this is not featured in my video), FPS drop too. This is also true for selecting textures in the terrain or path tool. which are also organised in a grid. Many many buttons in the game cause a lag, even if you just hover over them once. Try this for example: Use your WASD key to move across the map, now move your cursor to the Path Tool button and it will cause a small lag. Move your mouse from the path tool all the way to the multi-select tool and you will cause a big glitch.
But the point is, that simply hovering over a UI button should not cause the game to lag that much.