All those things get saturated much more quickly during high traffic times, whereas the freeway has substantially higher capacity to work with.

In particular most intersections on the now D809 are roundabouts, continuing on the D809 often requires making a turn on the roundabout, and roundabouts are notorious for gridlocking with high turn volumes. Let that gridlock cascade across multiple intersections and you now have rapidly deteriorating travel times.

At other times, traffic is less high so this gridlocking is less likely to occur.