I recently had an issue when entering certain chunks caused a massive lag spike, freezing the game up to several seconds. 

After trying to find the source of it, I along with fellow players concluded that the source of the lag spikes were a set of playerheads which had the names of non-existing players. 

Simply placing or removing these heads on their own would lead to great stuttering.

and the presence of several within a set of chunks would then always lead to the game freezing for a moment when loading set chunks.