I am attempting to slow entities in a certain radius around the player by a specific amount, including non-mobs such as arrows.  So far all of my attempts have failed, and as far as I can tell there isn't a variable in the Entity class that controls the speed of an entity.  Any ideas?

for living entities try entity.setAIMoveSpeed(0.5);

non living entities will be the hard part

you could try changing their motionX motionY and motionZ properties, it might work on some of them

 

Hm.... I might try that for living entities.  I tried multiplying the motionX, Y, and Z variables by 0.5D, but because of the method it's in (onArmorTickUpdate), it just slows them down to an infinitely small degree.  Any ideas of how to fix that?