It's based on Electron[1] which has a reputation for being a bit of a resource hog. The upside is that it makes it really easy to create a cross-platform app.
Slack gets pretty resource-intensive when the user is part of multiple workspaces. I know one of my co-workers is part of three or four, whereas I'm only part of the main company workspace. Co-worker's Slack memory usage is around triple that of my own.
