Why is rendering slow on an RX580 graphics card? From reading the release notes, it seems the app is mainly built for Nvidia cards, but apple has been using AMD graphics for years now?
-2010 5,1 Mac Pro, Xeon X5679 x2, 96GB RAM, RX580, Crucial NVMe SSD
Topaz Photo AI [v1.5.1] on Mac
Don’t expect too much from a more than 10 years old GPU. 
What file size are you sending to Topaz Photo AI for processing? If you are importing large files or using upscaling, then the processing time is expected to be higher.
This graphics card is mid-performance, not great compared to newer graphics cards…
If the processing is taking an unusually long time, open Topaz Photo AI, go to the menu bar on the top and click the Help > Open Log Folder menu option. Attach the logs files that you can see in the window that opens.
