All 25 modes checked one by one: three behaved incorrectly, four more silently collapsed into plain blending. 31 material and map classes closed. A 1,500-object scene went from "forever" to roughly 7 seconds.
Back to the Lab
Went through the blend modes and sped up heavy scenes
What's next
The journal keeps going
The other entries, one material taken apart node by node and an honest list of what still doesn't work are all in the Lab.