Node count doesn't matter. You could use an embedded database and encounter the same problem. There is some time T between when you acquire a lock and release it. Depending on the amount of contention in your system, this will have some affect on total throughput (i.e. Amdahl's law).

How familiar are you with MVCC?

https://www.postgresql.org/docs/current/mvcc-intro.html

Asking because needing a lock for changing a row isn't the only approach that can be taken.

Almost all commercial MVCC implementations (including Postgres) use row locks. Very few use OCC even though it's arguably a more natural fit for MVCC. Pessimistic locking is simply more robust over varied workloads.