Upgrade through the spawner GUI
Right-click your owned spawner, inspect its upgrades and read the current Essence cost before purchasing.
Spawn Speed increases production speed.
Critical Chance can double the logical output of a successful spawn cycle.
AutoKill processes covered production automatically.
Looting upgrades apply to AutoKill production.
Understand coverage
Upgrades cover a logical number of spawners. Adding more spawners can require extending or recalibrating coverage; the menu shows the cost for the larger stack.
AutoKill uses lower head-drop chances than manual kills. Choose it for the automation tradeoff rather than expecting identical head income.