After digging through the code, it appears we got the damageEffectScale setting backward. From line 37 in control.lua
local researchDelta = entity.prototype.max_health / (researchTotalCost * damageEffectScale) * (1 + attackingForce.laboratory_productivity_bonus)
we can see researchDelta (research gained per kill) is inversely proportional to damageEffectScale. So we need to set damageEffectScale to a big number if we want to reduce research speed.