Well, unfortunately I don't really know much about lxde, but here are a few suggestions...
First of, try creating a desktop icon as well and compare speeds, if the icon on the desktop and panel are both equally fast, and faster than the menu, it definitely proves that it is a menu issue. Also try loading them in different order, some applications run faster the second time because they preload.
Second, you could simply try removing the menu and adding a new one, though this isn't likely to do much.
Third, try creating a second user account and see if the error persists there.
Well... that's it for me, if none of the above works, I'm out of ideas.