Why deactivating GPU speeds up drastically LRC?

I have the same model (M1 Pro), but Lightroom Classic performs well for me. However, some of my specs are the opposite of yours: I have double the Unified Memory (32GB), and although I have two displays attached to my MacBook Pro, their total number of pixels is less than your single 6K display. When I use Lightroom Classic on the larger monitor only, the number of display pixels my M1 Pro GPU has to update are less than half of a 6K display like yours. 

So the GPU slowdown on your M1 Pro might be a combination of having a display with a very large number of pixels (6K is uncommon), combined with only having 16GB of Unified Memory. The reason the 16GB may be a problem is that on Apple Silicon Macs, Unified Memory is shared by macOS, any running applications, and the GPU. Lightroom Classic works best with more than 12GB available, so if macOS + Lightroom Classic are using most of the 16GB of Unified Memory, that doesn’t leave very much for the GPU so it may not perform well. Also note that in recent versions of Lightroom Classic, newer AI features and wider use of GPU acceleration are placing heavier demands on the GPU than when the M1 was released.  

I have decided that for any Mac I use in the future for Adobe applications, I will specify no less than 32GB of Unified Memory because that amount has worked so well up to this point on my M1 Pro MacBook Pro. 

If you want the best GPU performance on a 6K display, the next time you buy a Mac you might also consider one with more GPU cores (along with enough Unified Memory to feed it). The GPU in the Pro processor is near the bottom because it offers 10 to 20 GPU cores; the Max and Ultra processors offer from 32 to 80 GPU cores. Given the recent changes in Lightroom Classic, I might get a Max processor next time. 

Hi @ralphlouzon

Thanks for reaching out. Looking at your screenshot, I can see that you’ve selected the Custom option for GPU usage and enabled all available GPU options. This doesn’t mean GPU acceleration is partially enabled—in fact, it means Lightroom Classic is being forced to use the GPU for all supported operations. When GPU is set to Auto, Lightroom Classic decides how and where to use the GPU based on your system’s available RAM, GPU capability, drivers, and overall hardware resources. In this mode, Lightroom may use the GPU only for specific tasks where it’s expected to improve performance.

When you switch GPU to Custom and enable all options, you are essentially forcing Lightroom to use the GPU for all supported features, even in scenarios where the CPU might actually perform better. On some systems, this improves performance, but on others, it can introduce delays.