In etc/systemd/system.conf There is a shutdown delay of 90sec as default. But it should be commented out.
I take it that the 90 seconds wait is on the grey shutdown screen? If you hit ‘esc’ whilst on the grey screen it will give you an output that might tell you what it is stuck on.
when you power off your system the screen goes grey as it finishes up background tasks. (might be black for you depending on monitor settings) But seeing as you dont know what i mean it doubt that is where the problem is.