PostgreSQL performance degradation over time on a write intensive db
Primary and unique keys in all(?) RDBMSes use indexes in order to quickly be able to determine whether a newly inserted value is indeed unique.

The side effect of this is that queries via primary and unique keys are usually "fast".

Now if you haven't defined primary or unique keys on your tables,


You don't have a relational table but you have junk (OK, this is a contentious opinion, but a relational model needs keys on all tables).
queries on this table (in the absence of any other indexes) will become slower as more data is inserted into the table.


So yes, the absence of primary keys will cause this!