On an old PC the Unity desktop environment in Ubuntu might be slow and unresponsive. Here are a few tricks to make Unity Dash respond faster.
How to Disable Window Effects in Ubuntu 13.04
if you are still using an old laptop, you might be better off disabling all the windows effects and animations in Ubuntu 13.04 so as to improve the performance of the system.
How to Tweak And Perfect The Unity Desktop With Unity Tweak Tool [Linux/Ubuntu]
Unity Tweak Tool is a configuration tool for the Ubuntu Unity Desktop. It brings all the settings together in a single place so you can access them easily.