My Zorin OS 16 is booting very slow and after booting also it becomes unresponsive laggy.
You have several snap and flatpak packages installed. These can slow down boot time considerably.
You might consider decreasing boot time by disabling this unneeded service:
sudo systemctl disable NetworkManager-wait-online.service
You also can see what alternatives you might use to Snap or Flatpak in APT or .deb packages that are perfectly usable and fine to decrease the reliance on Snap or Flatpak that can inhibit boot and processing speed.
You can check for any bootup errors or system errors that may be causing lag:
Or to narrow it down to only recent boots;
journalctl -b -1
You might try the easy approach and reinstall the desktop to flush out any corrupted files that may be causing lag or sluggishness in Gnome-Shell and GDM
sudo apt update && sudo apt install --reinstall zorin-os-desktop
Lastly, on the subject of boot speed, this tutorial may be useful to you: