I would guess it uses less memory: it claims generally <100MB, although in my opinion that's still a ridiculous amount of memory for a desktop IM client to use. Still, it's better than 800MB.
Maybe at some point I'll find the time to revisit qweechat and see if it might be possible to strike a better middleground between "it's just chat" and all the fluff slack adds.
https://github.com/wee-slack/wee-slack/blob/master/README.md...
Still sucks, but better than the Web client IMNHO.