The slot system in Europe really stinks - but it seems be far better than it was before. When I started working in aviation about 7 years ago I was working as a redcap in ZRH
. We handled flights that went all over Europe. It was more or less standard that flights had 30-90 minutes of slot delay. There was very little coordination between different countries' ATC systems.
Now, due to a number of factors, it is far better than it was then. An example of something that has actually gotten better over time rather than worse
I am a pilot now at SWISS and we rarely have more than 15 minutes of slot delay unless the delay is at the arrival airport and it is foggy there. Then it is easy to have 60 minutes of delay.
I believe that Eurocontrol is better able to control flow these days and the introduction of RVSM has undoubtedly helped as well, though my plane is not RVSM approved and we max out now at FL280 - a seldom used altitude. We are lower than most jets, yet higher than most other turboprops.