Row deletes from Aria table very slow

DELETE LOW_PRIORITY QUICK FROM tablename WHERE col_name="XX" took 40 minutes to delete 16M adjacent rows from the front of a fully-indexed, ordered table with no other activity interfering with the operation. That seems very slow! (Many other operations, e.g. INSERT INTO WHERE (SELECT...), seem very slow also, so slow I keep checking SMART to see whether the disc is on the verge of collapse (smartctl claims it's not).