The DBPerformance log in logstash has entries like

Query affected 10002 row(s):
query-m: DELETE FROM `reading_list_entry` WHERE rle_user_id = 'X'

That's not so great. And could go up to 100*5000 in theory (someone with 100 lists with 5000 entries using the teardown API).

Deletion should happen in batches; also the API request should probably only delete lists, and cleanup of list entries should be left to a maintenance script (the code for which is mostly in place already, as soft-deletions are handled in a similar way).