I suppose the 4 hours saving comes from a lot of people being on the non-bridge route, meaning a lot of people choose to not take the bridge. Is there any other possible reason for the 4 hours saved?
You thing during particular holidays the single lane somehow has even less lanes, less grade separation and such? That would be quite a phenomenon.
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.