I was surprised because it felt so much cooler the second I went in.
こんにちは。
さまざまな言い方ができると思いますが、下記のような英語表現はいかがでしょうか:
I was surprised because it felt so much cooler the second I went in.
そこに入った瞬間に一気に涼しく感じたので驚きました。
felt so much cooler で「一気に涼しく感じた」ような意味になります。
ぜひ参考にしてください。
I was surprised how cool it was as soon as I got there.
It was amazing how cool it was the moment I got there.
この場合、次のような言い方もできると思います。
ーI was surprised how cool it was as soon as I got there.
「そこに着いた途端、そこの涼しさに驚いた」
ーIt was amazing how cool it was the moment I got there.
「そこに着いた瞬間、そこの涼しさにびっくりした」
ご参考まで!