I have installed this nice theme in a i7 machine with a 970 graphic card and works fine but if I enable the FPS counter I can see that moving through the game library can go under 30 FPS.
This usually happens because while you scroll through the library thumbnails need to be updated and images need to be loaded. The larger the files (snapshots can be pretty small, box art are way larger) the slower the update. On my Macbook I moved most of the snaps to a RAM disk to improve scrolling performance since the SSD was slowing down things a bit. Also consider that when you enable FPS count there's a hit on the CPU since the screen is continuously redrawn to calculate FPS, so don't keep it on all the time unless you like the sound of fans at max power

I tried it as well in a laptop with integrated graphics and even disabling all I could the FPS still below 10 and GPU usage 100 even if I'm not doing anything. Does it make sense to use that much GPU even when nothing is moving and no video playing?
Thank you for the heads up regarding GPU usage, I did not know you could check that in Win10 (I'm a Mac user at home, and Win7 user at work). I'll do some tests but 10 fps seems pretty low. What are the specs of that laptop? My MacbookPro late 2013 with integrated graphics card can do better than that and on i5 (2018) and i7 (2017) with integrated cards it works pretty well around 30 fps (but as I said, I'll check it better).
Now let's discuss performance and why AF is so demanding... Grid themes are inherently more demanding because they show and update more thumbnails each time you move to a new selection. AF in 2 rows mode usually shows 10 tiles on screen, plus 8 off screen as safeguard areas during scrolling. All the tiles need to be updated each time you change selection, and a couple of snaps needs to be loaded from disk when you move. I continuously try to streamline the code, reduce the need for updates etc but this is intrinsic in themes with many tiles on screen. For example if you turn AF to single row layout, it will be incredibly faster because the number of tiles is cut by half. On the contrary in vertical layout with three rows it's a bit slower.
When nothing is moving it should be less hungry but if, for example, you keep video snaps turned on then something is always moving and that will require resources (on my Macbook where AM doesn't support hardware decoding videos push the fans to hurricane blast mode

)
Now the GPU issue. I'll check with the resource monitor on Win 10, but AF uses GLSL shaders _a lot_ in a way most themes don't do. Shadows under the logo artwork are generated with GLSL, blurred backgrounds are GLSL shaders, there are multiple nested surfaces that can tax a GPU, plus glow effects, colorizing effects, some of the artwork cropping are done with GLSL shaders. For some reason even the visualization of letters in the scroll list and the visualization of genre icons, players icons etc use GLSL shaders to obtain some effects.
When I add some glsl shader to something I always check that the system is not severely affected, but with the growth of the theme I may have missed something, the resource meter in Win 10 will be helpful in this regards.
This long answer is not because I was offended by your considerations, on the contrary, I always try to improve AF, so please keep using and checking it, the feedback from the group was very important to improve AF. If you have poor performance let me know the specs of your system.