In order to get better performance on Xubuntu 14.04, we can enable the display compositing features on Xubuntu 14.04. A compositing window manager is a window manager that provides applications with an off-screen buffer for each window. The window manager composites the window buffers into an image representing the screen and writes the result into the display memory (wikipedia). On my own experiences, I cannot make the top bar transparent on Xubuntu without enabling this display compositing feature.
This tutorial is going to show you how to enable display compositing on Xubuntu 14.04.
First, go to Settings – Window Manager Tweaks. Next switch to Compositor tab and check the option enable display compositing. Close the window and now it should be enabled automatically.