In my experience, most problems caused by slots lie with the company, or sometimes slack discipline or operational difficulties at the departure airfields.
Can't blame Eurocontrol who can only go on the data they are given. Crap in = crap out.
In the London area, airborne holding is the modus operandii to ensure that there is a full reservoir of aircraft to provide maximum arrival capacity on the runways. Hence the UK AIC stating that 20 minutes delay in the air can be expected as the norm. A LTCC Terminal Control ATCO can probably explain the theory in more depth
