JFK is the region's largest airport in terms of land area and serves 47 million passengers per year in 2008 from eight terminals. From 2004 to 2007 taxi-out delays increased by 115%.
The United States Department of Transportation (USDOT) has instituted a cap on the number of scheduled operations (arrivals and departures) permitted at the airport at 80-81 per flights per hour, even though delays often push flights off-schedule, resulting in the actual flights per hour being considerably higher than the scheduled cap. There are other constraints aside from airspace that impact JFK's operations and its ability to expand.
The airport itself is physically constrained by the Jamaica Wildlife Refuge, a National Park and wetland, as well as by intersecting runways, various structures that can obstruct flight paths, and narrow alleyways between some piers (2/3) and terminals (4, 2/3 & 1).
@BetterAirports