Earlier quoted context omitted.
2GB of RAM use for 6 open tabs in Slack is pretty awful, their desktop client has set the bar to compete with Slack on performance very low. This abusive resource usage has created a dislike of Slack among a subset of its users.
I can honestly say that I have never checked or cared about how much RAM Slack uses. Or any other software for that matter. It's certainly not an issue that I'd use as the basis for deciding which team communication tool to use.
This is a pretty reasonable complaint when you're talking about a system with 2gb, and a chat application that uses 1.5gb of memory...or heck, a 16gb system when you have programs that legitimately need as much memory as you can give them and are being starved of it. Nobody would begrudge this usage if it was necessary for the task, but it isn't. Not even close.
Edit: Apparently this is better than it used to be, but there have been times when Slack also ate up CPU like crazy. Which, again, no biggie if you have plenty to spare, but when you don't...and even then, if you have plenty to spare, you're still going to be annoyed with Slack if it's eating all of your battery because the CPU is working really really really hard at...um...dunno? If it were mining bitcoin/monero without your knowledge or consent then at least it would be DOING something.