Yes, this is what the native macOS Messages.app does as well. On my Mac, with relatively heavy usage, I'm at around 100 MB of RAM.
Slack, on the other hand, feels like JavaScript frameworks piled three or four deep, often getting its UI out of sync with the actual state.
Chats/channels will briefly flash (1) unread in the sidebar to tell me about my own message that I just sent, or I’ll continue to have a blue “10 new messages” banner in a conversation that I’m actively participating in.
I use it because it’s convenient, but there’s definitely room for improvement.