Hello, I have same problem too. There is something on the official build because Parthas build is much, much faster.
Official GIMP build:
It takes about 38 seconds to open the image and draw it to canvas.
Adjust curves (without preview) takes about 1 minute and 34 seconds to complete the process and redraw canvas. CPU usage is about 30% and GPU usage jumps between 4-10%.
Rotating tool takes about 2 minutes and 20 seconds to complete the process and redraw canvas. CPU usage is about 70% and GPU usage jumps between 0-40%.
Parthas build:
It takes about 11 seconds to open the image and draw it to canvas.
Adjust curves (without preview) takes about 18 seconds to complete the process and redraw canvas. CPU usage is about 30% and GPU usage jumps between 4-10%.
Rotating tool takes about 20 seconds to complete the process and redraw canvas. CPU usage is about 70% and GPU usage jumps between 0-40%.
Also, there is no GUI lockups or hangs on Parthas build. For example, on official build when I’m on undo tab and click layers tab, or when I click Colors menu and choose any tool it may freeze GIMP for 10 seconds.
Test image for measuring was 42Mpix 16-bit TIFF. Using 8-bit JPEG was bit faster but not much.
My CPU is Ryzen5 1600, GPU is Radeon HD7700 series and I’m using Windows 7 64-bit. OpenCL was enabled on both builds.