Currently it is not possible to filter the relation endpoint. Hence you have to query all relations and filter the result, which can take a lot of time (up to several minutes).
Ideally we could filter on the relations via the querystring eg.
GET https://docs.denk-it.be/api/v1/relations?fromable_type=Asset&fromable_id=12345
or
GET https://docs.denk-it.be/api/v1/relations?toable_type=AssetPassword&toable_id=67890
or something like