WHERE...LIMIT much slower than WHERE...ORDER BY...LIMIT in MySQL 5.5
By adding the ORDER BY clause to your query, it resulted in the query optimizer choosing a different execution plan that uses the index to seek out the rows (which was a lot less rows to seek) as opposed to scanning the range of a lot more rows in your first query.
This isn't a guarantee that ORDER BY will always cause this to happen, but in this case it influenced the query optimizer to choose a different operation that happened to be more efficient (likely because the index it used is already sorted on the field you're ordering by).