When viewing the definition of an existing table, navigating to the SQL tab triggers DDL generation. Usually this operation takes a few seconds but in some cases where tables have a lot of columns and/or partitions, DDL generation can take > 30 minutes for a single table. The generation seems to have mostly finished after ~15 minutes, at which point the screen gets updated, though it is impossible to tell for sure because only the 1st screenful of the CREATE is visible and the "Generating DDL" dialog box still being up prevents scroll-down. It is noticeable that CPU is basically solid on a single thread for that first 15 mins (I have 4-core, 8 thread laptop), so 12-13% utilisation, then CPU drops to 0.
I am using SQL Developer 126.96.36.199.84 on Java 1.7.0_55