That's a good explanation of the added difficulty posed by the slot corner position. It always have been a fact; it simply has become much more significant in recent years. Not long ago, the only time the offense put three WRs on the field was on an occasional 3rd and long, or more likely when they were trying to play catch up. Now they may do it on any down and distance, and at anytime in the game. And so the 3rd receiver, the one in the slot has become a weapon not just a sub.
Correspondingly, the third DB, the guy we have referred to in an almost dismissive manner as the Nickel Back, has become a specialist as well. He's no longer just a Nickel Back or Nickel Corner; he's the Slot Back.
But we tend to think that the outside, the perimeter Corners have the toughest job because they may be covering the opponents best receiver. True, but when a corner is lined up on the outside, coverage actually can be easier. The opposing receiver is limited as to where he can go. He has the sideline constraining him. A good corner can use the sideline as a defender and pin receivers against it. And he almost always has a Safety behind him.
But unlike perimeter DBs who can often dictate the release of the receiver by their leverage - how far inside or outside of the receiver they line up- the slot DB needs to be quick enough to react to a “two-way go” from the guy he's covering. That takes a special kind of quickness and reaction speed that not all DBs possess. So indeed, the slot is a difficult place for a defender. A receiver can run up the field. He can stop. He can cut either way and frequently has an option built into his routes based on how he reads the coverage. That can require a lot of study, a lot of mental effort and concentration.
Slot receivers also line up behind the line of scrimmage at the snap, making it difficult for the defender to get a jam on the him. And because he's closer to the line of scrimmage, slot players have to be able to defend the run, and make tackles to prevent yards after the catch.
And of course, we have talked more than once about the trouble that so called elite CBs, Asante Samuel and Nnamdi Asomogha had in trying to play the slot, and also why Rolle rather than Ross had to play it for the Giants.
But does anyone reads this stuff before the flood of fluff and non-football threads pushes it out of sight?