That’s because Slack runs on top of basically Chrome, which is a horrible battery hog.

If you run the web versions of Electron “apps” in Safari you’ll get substantially better battery life. (Of course, still not perfect; irrespective of browser all of these types of apps are incredibly poorly optimized from a client-side performance perspective.

If large companies making tools like slack had any respect for their users they would ship a dedicated desktop app, and it would support more OS features while using a small fraction of the computing resources.

(Large-company-sponsored web apps seem to be generally getting worse over time. Gmail for example uses several times more CPU/memory/bandwidth than it used to a few years ago, while simultaneously being much glitchier and laggier.)