I would suspect a file structure issue or permissions. Do you have many apps that start up when you boot?
On my little Macbook Pro (no SSD) with 8GB RAM and a quad core 2.5Ghz i5 Archicad launches and loads the standard AC19 template in about 40 seconds.
I tend to run a minimal setup and only generally have Safari and Mail open.
Your Mac Pro has a few drive bays, I'd suggest clone you system disk and do a fresh install if the permissions repair doesn't work.
Having looked at the CPU usage monitor while launching AC I only use about 40-50% of one core, so you should not be lacking on the hardware side...
2012 13" Macbook Pro 8GB Ram, OS X 10.14.6
2010 Mac Pro 2x 6 Core 2.93Ghz Xeon, 48Gb Ram, OS X 10.14.6, RX 580
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