As a workaround we also can use –dbus command line flag and a systemd service. For example:
[Unit]
Description=Run spectacle in background
PartOf=graphical-session.target
After=graphical-session.target
Requisite=graphical-session.target
[Service]
Type=simple
ExecStart=/usr/bin/spectacle --dbus
Restart=always
[Install]
WantedBy=graphical-session.target
Restart=always is necessary because spectacle exits every time screenshot was taken. On the other hand, it’s 200MiB of used RAM (well, at least on my machine). Now it opens and takes screenshots instantly