Does this slow processing of SQL statements happen all the time, or only during some period of time after starting SQL Developer?
It's probably related to some extent to the amount of SQL History you maintain. In my case, I have about 1,000 *history.xml files in the SqlHistory directory. If you track CPU usage with Task Manager, for example, you will notice peaks during start-up, first connection open, and first SQL execution. It may take a couple of minutes before everything settles down to a lower level to permit consistent response times.