Most US airports have no slots etc . Airlines are free to add flights whenever they want to. Imagine what LHR would look like if that could be done? Same as JFK.
Problem is no regulation. And when there are, congress has a habit of tampering with them for anybody who makes campaign contributions or who promises to cut airfares.
Other than LGA, ORD, DCA and JFK there are no "slots" at any other US airports. No need for a "slot" to get a departure clearance. Airlines are free to operate what they want. Unfortunately they all want to add as many flights at the peak times as they can. If they do not, one of their competitors will and the delays will be there anyways. Anti trust regulations prevent them from discussing who can operate when. The little RJs allowed other airlines to offer flights on what were not productive enough routes for large airplanes. Nothing like being Number 40 inline at New York LGA and seeing that more than half of the aircraft ahead of you have fewer than 50 seats.
In the case of JFK it was necessary for congress to add lots of slots (as at LGA) for new airlines that promised low low fares to the voting public. So now there is a low fare airline in JFK with lots of flights. And there are lots of delays.
Also when foreign airlines want to add flights they let them have slots too. Unlike LHR where only 2 US carriers are even permitted to have slots. If they worked like this and forced the low fare carriers, new entrant foreign carriers to say, Stewart field way out in Newburg NY then maybe there would be less of a mess. If they could build a new airport to replace the fine one they have (well fine if you are in a DC 7 ) it would help. But unlike some places in the world, they cannot just go out and do that. Plus, we are spending all our money bring peace, freedom and democracy to the Iraqi people