Do you use a Plasma Wayland session?
No Wayland for me!
The issue happens specifically with Wayland. Looking around and from my own experience, it doesn’t happen with X11. I use a Plasma Wayland session on both my desktop and laptop with the only difference being my desktop is on Arch and my laptop is on Endeavour. I do not experience this issue on my desktop. Both should be using a very similar set of drivers. The desktop is running an Intel CPU + AMD GPU and my laptop is on an AMD Ryzen APU.
Shutdown on Wayland is the same barely 3 seconds. It has something to do with your hardware or other software on your system. Could be Wayland on YOUR hardware.
Edit: I have 4 other systems here. One running Arch, One running Manjaro, other running EOS. I can guarantee you all of them shut off instantly! One is a Lenovo Thinkbook 14" Ryzen 5500 also.
Lenovo ThinkPad E595. AMD Ryzen 5 3500U with Vega Gfx. Not exactly the latest bios version but close to it. I swapped the Realtek Wifi card for an Intel AX200. It was perfectly fine until a week or two ago. I haven’t tried downgrading my kernel like I said in a previous reply.
Maybe this would work for you?
Their workaround didn’t last as they said themselves. Properly looking at my shutdown and reboot logs, SDDM gets shutdown early in the process but decides to startup again later.
I could set a more aggressive timeout in the mean time like other people have done. Still slows down my shutdown and reboot.
The original didn’t last editing the /etc/mkinitcpio.conf so they created a shutdown script which did work.
Edit: At least that’s what I’m reading out of it. I don’t have the issue. 
It can also be worked around by logging out before restarting. It is definitely a wayland thing.
I possibly managed to fix it with some modifications in /etc/sddm.conf. I need to test further but I tried using the Wayland SDDM greeter and it seems to work. First important difference, no fail in switching TTY when opening the Wayland session. I am not sure the Wayland greeter is being used at all but it seems to work for now. I’ll have to try rebooting after somme activity to see if it is really reliable.
Edit:
What I did is similar to what this suggests https://bugs.kde.org/show_bug.cgi?id=445449#c23 .
Instead of using Weston, since I don’t have it installed, I tried kwin_wayland.
The config did nothing. I have found out something however : rebooting or shutting down from the interface (ie in the KDE start menu) within a Wayland session does not cause SDDM to slow down the reboot process. If I reboot or shutdown using the reboot or poweroff commands in a Konsole session within a Wayland session, then SDDM fails to stop correctly.
That’s the way i do it. 
Pardon me for preferring commands. Well, I guess I’ll mark it as solved. Use the UI.
I’m just saying because that’s how i was shutting it down. Not with commands so i wasn’t doing the same thing.
I use sudo shutdown now but should be the same than the other commands mentioned above.
shutdown is just a link to systemctl
$ ls -al /usr/bin/shutdown
lrwxrwxrwx 1 root root 9 Jan 29 22:01 /usr/bin/shutdown -> systemctl
I do just poweroff without sudo and the system shutsdown.
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My shutdown is just a click away… 
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Doesn’t it wait for 30 sec, or is it just because I use the now
No, I haven’t had any waiting for the system to shut down.