I've seen two writings of this on Duolingo and Jisho. Either as 速い or 早い. I believe both are read as はやい, but which of the two is more relevant in expressing how fast something is or how early someone arrives?
I can imagine 速{はや}い used in a sentence like this, 電車{でんしや}は速{はや}いです or Trains are fast.
