Many thanks for helping hunt down the issue. Updated my latest perf compare against the same stress scene in Seedborn in webgl2.
Kudos to @Evgeni_Popov for all the hard work put into frame graphs! 
For me, the primary lesson learnt: lights interaction with frame graph tasks work a little different from the usual scene.render case. I’m happy with how stuff turned out in the end, its really satisfying when the hard work pays off.