Discord is an Electron app, so while it's not as bad as Slack, it's still hardly what I'd call lightweight, especially in comparison to IRC/XMPP clients.

There's a lot to like in Discord, and it rapidly became the go to client for all my gaming related friends and activities, but it suffers from the same root pitfalls that Slack always has. It will inevitably run out of VC money (or whatever they're spending now) at some point and start wanting to actually turn a profit.