I notice how long it takes to start up the app, I notice how hard my CPU fan runs when Slack inexplicably decides to max out a whole core for a few minutes, I notice the data hit if I have to download a new version while on the road. It's definitely bloatware.
But I do notice and on my machine I prefer lightweight apps, so I can make the most of my existing resources instead of wasting them on a chat application.
My laptop battery sure does. Electron is legitimately wonderful, but apps built on it are often horrible CPU hogs.
If you're in more than one workspace it can take literally 5-10 seconds to transition between them while showing some BS motivational message.
Slack lags seconds behind when I type anything.
I actually ran into that today and restarting the app fixed it. That shows that there's a pretty serious limitation if such a high profile app has these sorts of problems.