Direct Opposites:
* Delay: To postpone or make something happen later.
* Procrastinate: To put off doing something, especially out of laziness or a lack of motivation.
* Slow down: To reduce speed or pace.
* Hold back: To prevent something from happening or progressing.
Other Opposites Depending on Context:
* Relax: To become calm and less hurried.
* Loiter: To stand around or linger in a place without a particular purpose.
* Hesitate: To pause before doing something because you are unsure or unwilling.
* Dally: To waste time or delay doing something.
The best opposite for "hasten" will depend on the specific situation and what you are trying to convey.