Why does adding LIMIT to my query make it crawl?

Question

Simple query:

select sum(score) total,name,gender,dob,country 
from users join scores on users.id = scores.user_id
where date between '2012-01-01' and '2012-01-31 23:59:59'
group by scores.user_id having sum(score)>=1000 order by sum(score) desc limit 50

So trying to get a list of accumulated scores for Jan 2012, order them by scores descending and paginate them.

Without limit: slowish but OK: searches 69348 rows. (Be nice to work out how to avoid the temporary table but I can't). Explain says:

1, 'SIMPLE', 'scores', 'range', 'user,date,user+date', 'date', '8', '', 69348, 'Using where; Using temporary; Using filesort'
1, 'SIMPLE', 'users', 'eq_ref', 'PRIMARY', 'PRIMARY', '8', 'scores.user_id', 1, 'Using where'

With limit: it's the same but the rows search are now 1806794 and it takes forever.

It's a partitioned InnoDB if that makes any difference, with all data being on one partition.