To be fair it really doesn't take that long, and often it's because it's auto updating, but it's not more then a couple seconds.

The big thing IMO is that once started I normally leave discord running, and most actions within discord itself feel very snappy - E.g. You click on a voice channel and you're instantly there. I think that's what they mean, they're trying to keep the delay for such an action low. Sometimes you click a voice Chanel and there's a few seconds of delay, those for some reason more annoying then the long (ish) startup time