Starting from version 4.0, you may notice SqlDeveloper "freezes" quite regularly; it simply does nothing and does not accept keyboard input or mouse clicks. It remains in that state for 20 or 30 seconds, then continues to work normally. No error message or log entry.
It seems to be a java bug. Updating NVIDIA video driver can improve the performance. The recommended workaround from the Java Bug DB -- set this property by adding a line in sqldeveloper.conf: