At my previous work we used Telegram. We almost never used video calls, but they're also available. CPU usage in calls is low (IDK for video calls). It has both

At my previous work we used Telegram. We almost never used video calls, but they're also available. CPU usage in calls is low (IDK for video calls). It has both desktop and browser version. Slack, at my current work, seems much less responsive, and doesn't support calls in Firefox.