This one is easy, maybe I’m the only person who didn’t realise these were different words until I saw them written down –
速い and 早い, read “hayai”. 速い means fast or quick, while 早い means early. Before learning their kanji I thought they were the same word, because if you get somewhere early you travelled quickly, right? In my defense I had just started studying, and I always forget how complex languages are when I’m just dipping my toes in.
Anyway, combine them to get 早速 “sassoku”, meaning immediately.
If you know radicals and need a way to keep them apart; 速い has that squiggly radical that means road, because roads are where fastness happens – such as on a 高速道路 “kousokudouro”, or highway if you will.
早い is made from 日 (sun) and 十 (ten), because for me it’s waking up too early if the sun is still out, let alone ten am.