You raise a good and important point. I think it is crucial to understand what happens when data is deleted. I've seen many applications, where this was not thought about until the storage was full or slow or the data protection people came round and complained.
DELETE rows in Oracle, the space is marked as free, but left as "holes" in the storage. If you
INSERT again afterwards, these holes might be filled if the new data fits in.
Some but not by far not all queries are sped up by indexes. Those that are not faster need then to read all data, including holes (so called full table scan). Reading an 8K block from disk that is full of data takes the same amount of data than reading an completely empty block which contains only "holes". Your time measurement points to this scenario.
Regarding statistics: Oracle updates the optimizer statistics at the nightly maintenance window (usually 10pm/22:00). Personally, I'd certainly gather stats after deleting a third of a large table, especially if this is a regular job, for instance in a procedure.
Regarding row movement: I leave it always on for most tables, but I'd judge it as not so important. It hurts only code that stores rowids in tables or longer time in memory and we don't do that. It is necessary if you do a
ALTER TABLE xxx MOVE.
Regarding shrink: I wouldn't be bothered to shrink or reorganize for 300k rows. For 10GB+, it may be worth the effort.