The traditional Penrhyndeudraeth route is 2.5km longer, the revised route adds 100m of climb.
Take your pick, time wise probably not much in it. those additional kms will take at least 6 minutes, 100m of climb probably costs about the same, but then much faster on the long downhill.
However, there is a spar shop in Penrhyn~ that is open until 11, likely the last open shop until Newtown for many people. If I got to the junction after 11, I would definitely go over the hill. If I got there before 11 and wanted to buy snacks for the 66km from Aberdyfi to Newtown then I would accept the longer route and the time lost.