fast强调速度
a fast car
一辆快速的车
a fast train
一辆快速的火车
You are talking so fast.
你说话说得太快了。

quick强调时间
a quick shower
快速洗个澡
We just need a quick answer.
我们需要你快速给个答案。
We should do it as quickly as possible!
我们要尽快完成