Slack is the worst I have to deal with on a daily basis. Fans always spin up on my T490 and it takes 10-15 seconds for all DOM elements to fully render. This is a laptop I have no problem compiling browsers on and it gets hammered by what could be replaced by existing XMPP clients that initialize faster than a blink of an eye.
Haven't tried this but it's a Slack alternative from the Basecamp folks, source included and works on airgapped networks.
Second worst offender I have to deal with is Jira web. I've had CTOs tell me they know it's crap but they force their startups to use it from the get-go because it makes getting acquihired easier because the other company likely already uses it. Truly Kafkaesque.