It's worse than that - if you just turn up for a DART at random your expected wait time will drop from 7.5 minutes to 5 minutes. So, statistically you'll only gain 2.5 minutes from reduced waiting time which will be offset by increased journey time after about 3 stops. When you factor in the decreased reliability the result is going going to mean massively increased journey times for everyone.
I did take this up with the NTA the last time they tried this as it also adds about 15 minutes to a day in Dublin for long-distance commuters on Sligo due to the earlier departure of the 0545 from Sligo and the later departure of the 1705 from Sligo.
They seem to have made a slightly better job of Sligo this time round as the later departures of the 1705 and 1905 seem to better reflect the reality of the journey - in particular removing a long wait at Killucan for the 1905 to cross with the 1800 from Sligo. They now have the 1915 down leaving Enfield at the same time as the 1800 up leaves Mullingar so sensibly the much less heavily loaded up train is scheduled for the long wait.
The 10 minute DART on present infrastructure is a silly stunt so that the NTA can be seen to be Doing Something. It is probably going to spectacularly blow up in everyone's faces especially once the first big storm of the autumn takes down a heap of leaves.