Postgres 9.3 - deletes terribly slow for 10 minutes after dropping indexes/constraints If I run a script which drops indexes (17 of them, CONCURRENTLY makes no difference except that without it I get the occasional deadlock), then constraints (20 constraints on 18 tables) and then run a function which deletes 100,000 records from a main table and all of its associated data from ancillary tables, the initial delete takes about an hour.

If I wait 10 minutes before running the function, it runs in about 16 - 20 seconds.  I run this function 120 times in separate transactions in this script, then rebuild my constraints and indexes at the end (postgres was hanging in the middle if I try to keep them all in the same transaction and even at 500000 per transaction sometimes resulted in a performance hit).

Is there anything which postgres does following index/constraint changes that could account for the need to wait on running my deletes?
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.