(But I realise I'm well out on the losing end of this argument, even in groups of old-school hard core technical friends/colleagues, I've been given the "crazy look" when suggesting we just use irc for chat... )
I'm not doubting you, and I acknowledge that I hear that kind of thing with some frequency, but I've not seen Slack (running in Chrome on Ubuntu) use anything like those amounts of resources.
I've had a pretty busy slack client running for several days, and it's using 204,484K of memory and 2.0% of one CPU.
USER PID %CPU %MEM VSZ RSS TT STAT STARTED TIME COMMAND
fubar 2922 0.0 0.1 61648 26668 2 S0+ 19Mar18 22:58.73 weechatRe: Slack resource utilization: I'm not passing a value judgment as much as expressing confusion over multiple claims I've noticed about Slack utilization that is far, far greater than anything I've seen.