As of 1.3-20, CPU lag is observed in WorldTickEvent:

• LagGoggle consistently report 0.5 to 0.6 ms per tick
  

• WarmRoast reports typically 0.5% usage
  

From a quick code review, every tick, for every entity item, a new Event is created and fired, even when no mod is registered on said event:
https://github.com/Vazkii/AutoRegLib/blob/master/src/main/java/vazkii/arl/util/ItemTickHandler.java#L22

How many entities were in the world at the time of this happening? You'd need a lot of entities for this to be relevant at all I reckon?

Probably around 4k entities of all types at that time.

Using VisualVM memory profiler, at least 35% of BlockPos allocations were referring to that custom event. There was ~1900 item entities loaded at the time.