MySQL - same queries, vastly different perfomance

wtf.sql

-- InnoDB table posts (uid, chan, board, threadId, id, ....)
-- primary (uid)
-- index (chan, board, threadId, id)

-- slow as fuck (Using where; Using index)
select distinct board
from posts
where chan = '4chan'

-- fast (Using where; Using index for group-by)
select distinct chan, board
from posts
where chan = '4chan'

-- fast (Using where; Using index for group-by)
select board
from posts
where chan = '4chan'
group by chan, board