I’d recommend that you check out Discord. Yeah it’s geared towards gamers, but it offers nearly all the same features Slack does for absolutely free. It consumes far less resources, has a much better invite flow, and a single account will log you into all of your servers anywhere you access the app. Oh, it also has built in voice chat, so there’s that too.
Need more convincing? Check out this article a friend wrote up about migrating his 4,000+ member Slack community to Discord about a year ago. Basically all of the drawbacks he lists at the end have been addressed since.
https://medium.com/@polymoon/unreal-slackers-is-moving-to-discord-c93492eb0612
After reading your article today, I did some quick evaluation of how Slack was performing on my Windows 10 machine vs Discord and these are the numbers I got:
Slack: 9 active processes, 662.8mb of ram, 6 servers
Discord: 2 active processes, 123.8mb or ram, 13 servers