East bound North Western to Northern SH1 is what I encounter every morning. As as you rightly point out, that on-ramp becomes its own lane so there is no traffic merging onto the Northern.
I usually meet the on-ramp queue heading north at Western Springs, takes about 30 minutes to travel the 3km distance from there to the Northern motorway. I've followed many drivers who will actually drive into the city to enter at Wellington street which simply moves the bottleneck from a 5 lane motorway to a single lane central city street.
Would an alternative of travelling down Nelson, and then turning left onto Fanshaw be any better ?
I get off at Nelson street every day and even on my motorbike, getting from the lights at the top down to the Viaduct is very slow. There are 4(?) sets of lights down Nelson Street and its always heavy in the mornings. Plus with the recent changes to allow for the cycle lanes (great move by the way, I used to cycle down there before the lane was put in, and its scary!) and left turn only lanes, people are always swapping.
Plus you have 2 sets of lights on Fanshaw Street before the motorway.
So may not be much faster.