Extracted from ubuntu forums:
[So I've been using XFCE and enjoying it thoroughly. I have two small issues that bother me. The first, when I right click to bring up a menu there is a slight pause as the menu comes up where the outline is there and filled with garbled stuff. And then the menu shows up. It's mostly noticeable with the main desktop menu, but also in just app right click menus. It's definitely not a deal breaker, but something I would like to fix.
Second, xf4run, when it pops up using Alt+F2, the text isn't highlighted. Again not a big deal, but something I would like to fix. Oddly enough, the first time I open it up when I start XFCE the text is highlighted. It is just afterward that it isn't.]
Extracted from xfce wiki:
[It is more likely that the icon theme you are using renders too many SVGs making it very hard to scroll. Switch to another icon theme.]