postgres Poor performance on ORDER BY "id" DESC LIMIT 1
Trying to explain why there is difference in performance between the two queries.

This one: SELECT * FROM "items" WHERE "object_id" = '123' LIMIT 1 is satisfied by any one row with the matching object_id, so the index on object_id is a natural choice. The query requires minimal I/O: index scan to find the first matching value plus one heap read to fetch the entire row.

The alternative: SELECT * FROM "items" WHERE "object_id" = '123' ORDER BY "id" DESC LIMIT 1 requires all rows with the matching object_id be sorted by another column, id, then the row with the maximum value of id be returned. If you were to use the index on object_id you would need to perform the following operations: scan the index to find every matching object_id; for every match go fetch the actual row; then sort all fetched rows by id and return the one with the largest id. 

The alternative chosen by the optimizer, presumably based on the object_id histogram, is: scan the index on id backwards, in its entirety; for each value go fetch the row and check if the value of object_id matches; return the first matching row, which will have the maximum possible id value. This alternative avoids sorting the rows, so I guess the optimizer prefers it to using the index on object_id.

The presence of an index on (object_id asc, id desc) allows for yet another alternative: scan this new index for the first entry matching the supplied object_id value, which by definition will have the highest id value; go fetch one matching row and return. Obviously, this is the most efficient approach.