During the draft telecast, Joel Klatt made a point to say that this concern was largely unfounded. He said that many analysts don’t understand the Ohio State offensive scheme. He noted that they run a read route offense where Fields is required to hold the ball longer.
Ryan Day also went through some examples where Fields went through 3,4,5 reads. I realize he's his college coach and will probably be his biggest advocate, but after watching more tape on Fields, I'm not concerned about this idea that he can't process his reads.
It's true that he holds the ball for a while and that his thought process will need to speed up to NFL levels, but that's true of any college QBs going to the pros. I'm not any more concerned than I would be for any other good college quarterback going to the NFL.