it seems you’re index has seriously low cardinality. in terms of MySQL it shouldn’t use index at all, it’s easier to scan table directly. If you’ll add a hint to your MySQL query to mandatory use index on ‘type’ field you’ll get same performance degradation.
My recommendation would be to drop this index at all.
P.S. ‘usually’ in RDBMS systems query optimizer turns on indexes when less than 10% of data will be selected.