I'm curious about the claim of "fewer notifications" I'm not really sure that I see how a new chat app achieves that. Would you be able to expand further?

we don't send notifications for @-mentions, by default. instead, mentions ends up in your activity feed so you can check it on your time. But, for when you need to reach someone, we have !!-mentions — which will send a notification.

(you can also set per-channel and per-thread notification settings — so you can turn it down, or turn it up.)

On the Linux "app" it appears to be Electron, but it's somehow a lot more responsive than others electron apps. Any comments on this?

It is indeed Electron on Linux, but we're not doing anything special to make it more snappy on Linux.