I realize this thread is a little old, but I recently ran some tests on the effects of the GPU on Gigapixel AI performance that I though might be of interest to some who are grappling with performance issues. Might as well keep such info in one place I guess.
Unless you put GAI into cpu mode, how it runs is very much dependent on your GPU. I have two Windows 10 computers, both with 32GB of fast RAM and near top of the line CPUs at time of purchase. But computer1 has a 3.5 year old Nvidia 960 2GB GPU, and computer2 has a 6 month old Nvidia 1080 6GB GPU.
I just ran some tests using several images that are 4608 x 3456 px. Using GAI to upscale by 400%, computer1 took about 7:08 (minutes/seconds) to completion. Computer 2 took about 3:07, or less than half the time.
I then took the newer GPU from computer2 and put it into computer1, and ran the tests again. Computer2 then took about 1:36 to finish the same tasks. That's a 78% reduction for computer2, entirely due to the newer, more powerful GPU. Very interestingly, with the new GPU, the older computer1 ran AIG considerably faster than the newer computer2 did when it had that GPU. So there must be some important other differences between the computers that also have a bearing on GAI processing time.
Besides Gigapixel AI, I also ran similar speed tests on Topaz Denoise AI and Sharpen AI; the latter program in all 3 modes of Sharpen, Stabilize and Focus. The newer computer/newer GPU combo was in each case about 3-4 times faster than the older computer/older GPU. But for these programs and modes, when I put the new GPU into the older computer (older computer/newer GPU), it reduced processing times to about the same level as had been the case for the newer computer/newer GPU, but no lower. So apparently the differences between the other components in these two computers had little impact on DAI and SAI, and the faster processing times were entirely due to the new GPU.
Computer1 + 960 2GB GPU
GAI ~07:08
DAI ~00:48
SAI sharpen mode ~01:32
SAI stabilize mode ~04:10
SAI focus mode ~04:10
Computer2 + 1080 6GB GPU
GAI ~03:07
DAI ~00:15
SAI sharpen mode ~00:20
SAI stabilize mode ~01:03
SAI focus mode ~01:02
Computer1 + 1080 6GB GPU
GAI ~01:36
DAI ~00:13
SAI sharpen mode ~00:20
SAI stabilize mode ~01:00
SAI focus mode ~01:00