If you always respond instantly you set that expectation for the future. I wouldn’t be expected to respond immediately if I were presenting over Zoom, had just left for a bathroom/drink break, on an important call, or was in the middle of an careful process anyway.

It’s not as if I ignore it completely, I can check Desktop 1 for flashing notifications with a simple left hand only Alt-1, and be back on my working desktop in like a second. That quickly becomes as autonomic as Ctrl-l, n, e, Enter when I’m in a browser.

Really depends. At our company there's no expectation that you'll answer Slack messages in real time.

IMHO expecting synchronous behaviour from asynchronous tools is flawed way to operate; I'd be very comfortable taking a stand against manning these communication channels at all times.