If you’re willing to “block” normal access to the bucket for the duration of the delete you can use Flush operation to delete everything from it.
Alternatively you could create a Map/Reduce view which just emits the ID of all documents and query that - it may or may not be faster than N1QL.
Thirdly, if you just want to have all data disappear after a certain time (and don’t need to read it before it goes away) then you could set TTLs (expiries) on your documents.