also: さっきゅう
An immediate response is necessary.
この問題は早急に解決しなければならない。
This problem must be resolved urgently.
早急のご返事をお待ちしております。
We look forward to your prompt reply.
Please respond promptly to the complaint.
It was decided that immediate measures would be taken to solve the problem.
There is a need to improve this immediately.
We kindly ask for your prompt response.
早急 is a stiff, business-register word for needing prompt action, whereas 大急ぎ is the everyday word for personal haste.
早急 describes something that must be done immediately (as a な-adjective), while 急遽 describes the act of doing something hurriedly.