Well, let’s take one of the problem apps; Slack. It is slow because it eats a lot of memory and makes my computer slow. You are right that networking makes apps slow, but not to the point they feel slow now. If I have a background thread getting Slack messages and nothing came in, why is the entire thing slow as molasses? I know most apps/applications simply do 1-1 page to web api req and if the web api req is slow they give up (usually with some stupid message that they cannot connect / reconnect) but why would that be slow; you can do that blistering fast with win32. And yet my Slack is usually stuck and sluggish?

There is a lot of laziness going on: not such an easy relationship between slow web and frontend as you describe. Most apps can work fine with stale data for most uses; they just do not because then the developer has to think about it and that costs money/time and takes talent.