Have you noticed there is a delay autocomplete suggestions when you type in a variable name or a function name? My typing speed is decent and I found the code assist delay to be too much. By the time Zend Studio was ready to show me the suggestions, I had already typed the variable name.
By default, Zend Studio has a 200 millisecond delay in showing code assist. You can shorten it if you feel the delay is too long. I set it to 100 milliseconds.
Go to Preferences -> Java -> Editor -> Content Assist -> Auto-Activation and decrease Auto activation delay from 200 to 100 (or whatever suits you better).