notify-send messages sent from a systemd unit delayed

Question

There's an active systemd service

[Unit]
Description=Launch the screenlock daemon service
PartOf=graphical-session.target
After=i3wm.service

[Service]
Type=simple
ExecStart=env screenlock-daemon
Restart=on-failure
RestartSec=5

[Install]
WantedBy=graphical-session.target

that starts the daemon script screenlock-daemon:

#!/usr/bin/env bash

xss-lock --session="${XDG_SESSION_ID}" --ignore-sleep --notifier=notifier --transfer-sleep-lock -- my-locking-command

As per xss-lock, prior to locking it notifies user by running the notifier script:

#!/usr/bin/env bash
echo "executing at $(date +%R:%S)" >> /tmp/screenlock-notifier-exec  # message is logged at 
                                                                     # the expected time.
notify-send --urgency=normal --expire-time=10000 -- 'Locking screen soon...'

Problem is, the notification pop-up is shown with a multi-second delay. Only if a manual notify-send command is sent, it's shown immediately.

Is this something to do with how systemd works, or maybe it's the notification daemon I'm using - dunst?

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Running debian testing.