I’m talking about the native slack app. I used to use the irc gateway and now go back and forth between desktop and unofficial gateways. I use keyboard shortcuts to navigate between chat windows and nothing about it is instantaneous...even when there’s nothing but text. Same thing applies to discord, and less so with WhatsApp and signal. Messages on the other hand is silky smooth, and terminal irc couldn’t get any faster.
Obviously if you build a chat client in the browser and communicate between windows then it's instantaneous. Faster than human perception.
Certainly doesn't seem to be a CPU issue to me. I mean, we run videochat in the browser which works fine. Obviously text chat can be instant.