Sure, which was one of the options considered in my post. The 20180913 build with the current ffmpeg HEAD (91943-g1b98bfb932) wasn't out when I tested, so I couldn't be sure.
Still, to double performance in just about 8 days (Zeranoe aom revision: 20180906-5d963cb) is really a LOT, especially considering a lot of the commits are either bugfixes or encoding-related.
Turns out the commit that really made the difference is
f820da0 - Turn on the row-based multi-thread decoder by default.
Code:
# time ./aomdec.850e126ac.exe --threads=4 -o /dev/null "Gus Kenworthy & Tom Wallisch X Games Slopestyle GoPro Preview.1080.ivf"
real 1m0,929s
user 0m0,000s
sys 0m0,000s
# time ./aomdec.f820da02b.exe --threads=4 -o /dev/null "Gus Kenworthy & Tom Wallisch X Games Slopestyle GoPro Preview.1080.ivf"
real 0m37,947s
user 0m0,000s
sys 0m0,000s