Except, in my experience, they don't have more features. Has Slack fundamentally changed in the last years, or Zoom, Teams etc? Why haven't these apps been optimized by now?

Same for web apps, why is Facebook, Gmail slow? I could accept it for new products but not for nearly unchanged products after all these years.

Btw: Gmail HTML was fast, so it can't be an infra problem, it's bad frontend programming.