Actually ORD is much much worse than EWR. ORD is listed as #12 on the list of airport slots used, but EWR is #39. It might be the top 50 airports are considered slot restricted to make it easy.
ORD has 8 active runways
EWR has 3 active runways
pretty easy to figure out which airport will have more slot/congestion problems.
all airports in AWS will have slot restrictions because its the dynamic of the game, even though most american airports do not have a 24 hour slot restriction program in place (only at peak hours at busy airports). Just dont schedule 7am flights, problem solved.
Why is it LHR is so slot restrictive? probably due to the fact it carries hundreds of millions of pax and only has 2 active runways.