Whenever I visit Gmail, I see its loading indicator for a fraction of a second before it loads:
Here’s what it looks like when I artifically make my network connection dial-up speed, preventing Gmail from loading within a reasonable time (it would take several minutes for Gmail to actually load, but you couldn’t tell that from the loading bar):
Some investigation reveals that Gmail’s loading bar isn’t an actual loading bar! In fact, it’s progress is controlled by a CSS animation that makes it start fast, then slow down, then just stand still until Gmail is loaded. This defeats the point of a loading bar: it’s supposted to give an estimate of progress, not fill up at a rate entirely unrelated to actual loading!