Formal:
* With all due speed: This emphasizes the urgency of the task.
* With the utmost expediency: This implies a need for efficient and rapid action.
* As soon as practicable: This emphasizes practicality and feasibility.
Informal:
* ASAP (as soon as possible): This is a common abbreviation used in emails and messages.
* Right away: This implies immediate action.
* Immediately: This emphasizes the need for promptness.
* In a hurry: This conveys a sense of urgency.
* Double-time: This is a colloquial expression that suggests speed and haste.
Other options:
* At the earliest opportunity: This is a formal way of saying "as soon as possible."
* With all haste: This emphasizes the need for speed and urgency.
The best choice for you will depend on the specific situation and the level of formality you need.