Endermen spawn and accumulate in massive and endless numbers in dark areas of overworld in amplified worlds (possibly other world types, but I am playing amplified). These Endermen are in such huge numbers, outnumbering all other mobs, that you need not seek them out to farm ender pearls. They do a great deal of damage to the environment as well. These Endermen are literally a plague. I believe they maybe causing a lot of the lag spikes since they are constantly causing chunk updates.