Can somebody please explain to my boss that using a progressive loading bar when you've got no metrics to base the progression on is stupid. Pls.

Well, if there is some loading going on, but you don't know the progress, users may actually feel better if there is a loading bar instead of a loading animation like those wheels, because it feels like you know when it will end.

From a technical perspective still stupid, but maybe better UX.

I had a client like this. Told them I could do it if they wrote an API to provide the values.

They did. It goes up to 30%, back to 0, then to 20% where it sits before going to 100%.

They are happy. The invoice is paid. Only users got fucked.

Just do the windows way: slowely load based on a randomnezz and when its finished just load completely.