Looking at
It looks like the final section of the Drummochter climb (the dual carriageway) was the turning point.
But the schedule doesn't appear to allow for it much, when compared to the other 2 rides with enough datapoints to see the time loss up there.
The Luncarty-Pass of Birnham roadworks don't appear to have been an issue either, although they are fairly free flowing, at night there wouldn't be too much traffic to get held up by (mostly a 40 limit, quite narrow lanes and there's a section with a 30 limit and mini-roundabout).