Immediate
Instantaneous
noun, の adjective
An instant sit is immediate. It's also instantaneous. Just imagine yourself sitting instantly. No time passes between when you were standing and sitting. The instantaneous nature of your sit is definitely... troubling.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
即座の結果は期待しないでください。
Please don’t expect immediate results.