The reading for this one is strange. The reading for 速 (そく) is what you'd expect (on'yomi), but 早 gets a reading you haven't seen before, meaning you should treat this as an exception. Just think about how you're learning this word at once and without delay so there's no time for any sass (さっ), just kanji learning.
I used it immediately.
Thank you for your immediate response.
I will send the eggplants to him at once.