What I meant is, even if processes had no cost, you will probably bottleneck on the I/O. So it doesn't matter a lot how many Postgres connections you have.
Sure, it’s not the most common problem to experience, just something that’s useful to know once you have more and more clients. It did happen to me, all connections used up for long running transactions (for multiple reasons) and then nothing else was able to run, CPU and I/O nearly idling at <5%, no significant lock contention.