Unpredictable performance would mean, you can't predict how long a specific query will take. This is the opposite of how postgres works. If a query is slow, it's predictably slow if the data doesn't change. So what you must mean is, when the data changes, it doesn't behave like you expect.

So again, how do these poor choosers make better decisions in the face of changing data when they have to write all the algorithms by hand, and understand how to scale the data correctly? Seems to me that pretty soon they'd just be writing a very crappy database on top of some keystore.