slots
The slots can also be of many "hidden" reasons. If you transit several regulated sectors there will be a slot issued only for the most limiting sector. The slots in the other sectors then follow automatically because of your filed speed - in short the traffic in the less restricted sectors are slotted around you to allow you to reach your most restrictive sector on time. Since this is the same for all acft you can imagine that this can get incredibly complex and probably not even be close to possible without computers running ....
So the official reason will be the most limiting sector but that doesnt automatically mean that you would get a lot less delay even if the problems in this sector mysteriously disappears....