This allows to distribute AI reaction calls over time.
Before this change actors appearing at the same frame will react in the same
frame over and over because AI reaction period is constant. It creates a
non-uniform CPU usage over frames. If a single frame has too many AI reactions
it may cause stuttering when there are too many actors on a scene for current
system.
Another concern is a synchronization of actions between creatures and NPC.
They start to go or hit at the same frame that is unnatural.