spawnRateModifier, although honest, is a bit inaccurate. With this name, it implies that by increasing the modifier (default of 1.0) then the rate of spawns would also increase. This is not the case as per the comment:

# This multiplier is applied on the spawner's spawn delay each time a mob is spawned. 
# Can be used to make mobs spawned exponentially faster or slower.

Kudos for the documentation, but we can do even better.

This configuration should instead be named spawnDelayMultiplier or something similar, indicating that by increasing the value that the delay is increased, rather than the rate of spawn.

Bonus points if the documentation comment also indicates the direction (e.g. "Increasing this value increases the delay -- amount of time -- between spawns generated by this spawner.")