There are many bad things to say about Slack, being slow is a popular one. But even slow apps can behave decently if they really want to. Not Slack.
You see, because it takes so much time to start up, I usually start it in background while I’m busy with something else. But as I’m focused on my work, Slack Spinned #1 shows up, on top of everything I was doing at the moment. Loading spinners are usually a good thing, but the idea of them is that they are shown immediately to inform user her request is being processed. Not in Slack. In Slack, even loading spinned takes significant time to show up, usually multiple seconds. By that time you’ve completely forgot you started it and are already switched to something else. And here it is, interruption you and stealing your focus.
Ok, one mistake wouldn’t be so bad, but Slack team didn’t stopped here. As you’ve switched away from that spinner (there’s no point watching it spin for another ten seconds, is there?) and busy with your life again, time passes and Slack is finally ready to render a window. Of course, that window overlays everything you were doing AGAIN, and of course it has nothing to show to you but another spinner. It’s not ready, but it’s interrupted you twice already. I wouldn’t expect less from Slack.
So you switch away, again, and this time it will silently finish loading in the background and there’s no way to know when it’s ready for you. Just when you might (arguably) finally want it.