I'm comparing it to applications that may have somewhat more reasons for slow startup times, like mentioned below in the thread (think: Photoshop, Maya, Unreal Engine, etc). Not a chat application.
I'm not saying Slack is not full-featured, but it's definitely not on the same scale as some of these other applications.
Maybe, but I would actually argue with the assumption that an image editor is somehow inherently (allowed to be) “more complicated and/or slow” than a productivity app.
Or, more simply: Slack (and other similar products) are not just “chat applications”.
No, they're certainly not just chat applications, but if you think that Photoshop is an image editor that is somehow at the same level of complexity as Slack I'm not sure what to tell you. We'll have to agree to disagree!
Because very very little happens in the client itself. Basically any button that does something is just an api call out to their servers and an update to the view. You can see this yourself when you're working on a higher-latency network connection -- damn near every click gets you a spinner or a pause of some sort before things happen.
I'm not saying Slack is not full-featured, but it's definitely not on the same scale as some of these other applications.