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As far as I understand Spotify does not use Electron, it uses the Chromium Embedded Framework.

You are correct: https://www.spotify.com/us/opensource/

(OP here) I adressed that caveat in the late part of the answer. I also noted that I don't even think the performance difference is attributable to immediate vs retained mode. This was just a comparison agains existing applications.

As far as I am concerned though, it is pretty irrelevant to the user if an application is slow because the developer made a mistake, or because they wanted it that way.

It's fair because they're both GUIs. It shouldn't have to be anyone's problem that a set of developers chose to pack a whole browser inside of their user interface.

If the question is about comparing the performance of immediate mode and retained mode, the comparison should be between two otherwise-similar applications. There are a lot of applications that are faster than Spotify, but that says more about Spotify than about those applications.