This has been annoying me for a while on EL8 (CentOS / Rocky / Alma / RedHat). It actually affects more than just Slack – it can affect most things that send desktop notifications, but Slack is where I first noticed it. I haven’t been able to find a proper fix for it, but here is a workaround:
sudo rm /usr/share/dbus-1/services/org.kde.plasma.Notifications.service
Notifications that used to hang the app for minutes will now not go off at all and everything will continue smoothly.