After clipping the subject from the background I kick out a PNG on transparent with a "save as" action I created and have been using without issue for a couple years. I'm saving the PNG at 100% width and height, no interlacing, compression set to 9 (smallest file size). I get it will take time to compress the file, but there is a noticeable difference after the latest update. It would typically take 60-90 seconds to finish, but now I'm looking at 4-6 minutes. It seems like my graphics card is getting taxed to the point where my 2nd display (a Dell 24in 1080HD display) gets kicked off and on.
I'm running High Sierra 10.13.6, on an iMac: 4 GHz Intel Core i7, 32GB RAM, AMD Radeon m290X w/ 2GB graphics card.