This one is also interesting; written in 2012, it claims that Amazon could lose 1b+ from a 1 sec slowdown: https://www.fastcompany.com/1825005/how-one-second-could-cos.... I imagine people are even less tolerant of slow pages today.

Fixing website performance can be one of the cheapest ways to increase conversion because it's hard to figure out what else moves the needle.

Over-optimizing is not going to help you at all but if you ignore it eventually it will all seize up.

You have to keep that stuff in check.