"Dear Paul, Thank you so much for your rapid response to my email. I did't expect an immediate reply and it was really thoughtful of you to include all the details I need for my forthcoming trip to Japan! i am really looking forward to meeting you and hope to take you out on the town to try all the local beers! See you soon, Regards, Mike
例
"Dear Paul, Thank you so much for your rapid response to my email. I did't expect an immediate reply and it was really thoughtful of you to include all the details I need for my forthcoming trip to Japan! i am really looking forward to meeting you and hope to take you out on the town to try all the local beers! See you soon, Regards, Mike
ポールへ、メールにすぐに返信してくれてありがとう。そんなにすぐの返信は期待していなかったのだけど、今度日本へ旅行する時の詳細な情報を教えてくれて君は本当に優しいね。君に会うのがすごく楽しみで、君を連れて地ビールを飲むのが楽しみだ。もうすぐ会えるね。ではでは マイク
こんにちは。
quick reply や prompt response と表現できます。
quick、prompt は「早い」「迅速な」という意味で、reply、response が「返事」です。
例：
I quickly replied to the email.
「メールに即レスした」
Thank you for your prompt response.
「早速のご返信ありがとうございます」
ぜひ参考にしてください。
A reply can also be called a response, answer, or acknowledgement. Synonyms for quick include: swift, prompt, fast, speedy, and expeditious.
In a business setting or when writing business emails the phrase "expeditious acknowledgement" is very formal.
The phrase " quick reply" is more common. It also can be used for business settings just because it is so commonly used.
Thanks for the quick reply. 即レスありがとう。
というように使えます。
Wow, that was quick. わ、早い（レス）だね。
すぐに反応がある、迅速な、というのがquickのイメージです。
ササッと、という感じですね。
朝が早い、という時の「早い」はearlyです。
使うニュアンスが違うので、注意してくださいね。
1. quick reply＝即レス
I received a quick reply!=即レスをもらった！
2. To reply quickly=早く返信する
I am happy if you could reply quickly.=早く返信してもらえると嬉しいな
1.が「名詞」で使いたい場合で、2.が「動詞」として使いたい場合になります。
"Thank you for the rapid response!"
"Thank you for the quick answer!"
"Thank you for the swift reply!"
"Rapid" is a different word for "quick" or "swift"
"Response" a different word for "reply" or "answer"
“Thank you for the rapid response!”
“Thank you for the quick answer!”
“Thank you for the swift reply!”
“rapid”は、”quick”や”swift”と同じ意味で、速いという意味です。”response”は、”reply”や”answer”と同じ意味で「返信」という意味です。
There are many ways this could be said.
If someone is rapid it is done quickly and a response is an answer or reply so a rapid response is an answer or reply that is given quickly.
Swift is another word used for quick/fast. You would also use the word prompt meaning efficiently or in a good time/quickly.
イギリス 
アメリカ合衆国 
日本 
チリ 
南アフリカ 
