If anything, you should install an adblocker. A single website filled with ads (and they're all filled with tons of ads) can spin the CPU to tens of watts forever, significantly draining the battery.
CPU tends to be quite lean, until something needs to be done then steps up very quickly to consuming 45w.
It's quite variable, the highest brightness can consume double of the lowest brightness for example. One interesting test if one has a battery app showing instant consumption (I know lenovo laptops used to do), is to adjust brightness and see the impact.
Turn off animated gifs and emojis in slack.
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.)
Amen