Postgres 9.3 - deletes terribly slow for 10 minutes after dropping indexes/constraints
First, sorry if I have a bad English. 

Even if you dropped the constraints from the table you are trying to delete, the constraints of any children tables will affect the performance of the parent table. This occurs because the database needs to do an select on all the foreing keys pointed to the main table each line of delete to guarantee the integrity of the deletion that you're trying to do.

Create indexes on the foreign keys of the child columns that are pointed to the main table you are trying to delete, this will solve the problem and improve performance for inner join also. Remember PostgreSQL will not create indexes for the foreign keys automatically.