Back in 2000, there was the "3 second rule": web pages should be fully loaded within 3 seconds. At the time it was very much optimization of image size as loading was pretty much serial, with no async HTTP requests.

It saddens me that we have become so complacent; sites like Jira take 5 to 8 seconds to load a page that feels like it should be near instantaneous, and we're supposed to be OK with that. Under the hood, these sites are a spaghetti of nested API calls that probably make a lot of sense to the people owning the products but severely affect user experience.