On edit: I just noticed that the men's solo SUP time beat the men's solo canoe time. Sounds embarrassing to me, but I'll resist all petitions to change this site to SUPtripping.net.
The river has fast current generally everywhere, but with significant variations across its width. Moving just a boat length left or right may gain you as much as 3 or more mph. In general you want to be on the side of the river that has high vertical banks, but those can switch left to right around every bend. You can't efficiently keep traversing left to right and vice versa to chase and stay in the main current, and hope to make good time where the river can be nearly a half mile wide. Much depends on how skilled you are in choosing certain shortcuts on your route, especially in the highly braided multiple island sedtions of the Yukon Flats.
The river will determine where you go if you let it. Decide a half mile prior to an island which side you want to pass because the current will split before you get there. Read the surface riffles to see where this happens. Get in the wrong siide of current and you can end up traveling a mile or more farther than you otherwise might if you navigated better. Once you are captured in a wrong side split current it is very difficult to exit to a better choice.
Fast water enetering a narrow sluce may be temping to enter, but if it broadens out much a short distance later, you may find yourself in dead water with almost zero current to push you along.
Below Dawson (on the 1000 mile race) where the river tends to be broad with many meanders, you will get into spiral current flows, where a surface current develops that will whisk you to the outside of a bend, perpendicular to your desired course, significantly increasing your distance and time to travel forward. it can take sprint strength effort and prior knowledge of how to stay out of this devil current and to continue on a shortest distance fastest time path down river. It is all part of the strategy that I love about racing the Yukon.
It definitely helps to have prior experince on this river, to make better choices the next time.
This may be how the SUP did so well.