The Librem 5 kill switches work immediately without rebooting the phone or anything (sometimes the software takes a second to detect and display the change though, it depends on where the polling interval is when you trigger the switch).
I routinely use the WiFi kill switch when I leave the house to save battery by having it be completely off and not scanning networks I pass by. I could do it in software too but it’s actually faster and more convenient to do with the HKS than going into Settings at the moment.