Nasm Nasmus | Suffocating wither skeleton farm for 1.9/1.10 (5.5 skulls per minute) @nasmnasmus866 | Uploaded 8 years ago | Updated 20 seconds ago
This is my design of a wither skeleton farm that produces 5.5 skulls per minute.

World download:
mega.nz/#!RY5kjY7J!otRKAR_TL_iBPWjcszqXOqTz4NW9bPM5eVfcQz1BNJE

the farm is at 700,-1100 in the nether

Gnembon's video about subchunks and stuff:
youtube.com/watch?v=IF6Umbf28Y4

Similar design by reddit user BaccoShow:
reddit.com/r/Minecraft/comments/44w65m/wither_skeleton_farm_19_tileable_potential

A faster wither skeleton farm by ilmango:
youtube.com/watch?v=ibdb7c7LHAA