You have to keep in mind that processing an image is done by both CPU and GPU. It also depends on the noise reduction you’re using. And stressing out a CPU and/or GPU doesn’t necessarily mean that things go faster, just that your CPU and GPU can’t run any faster at that moment.
If I recall correctly, most of the processing in DxO products is done by CPU. Only DeepPrime (and the newer DeepPrime XD in PL6) fully utilize the GPU.
This graph is from processing 5 raw images with DxO Standard profile and DeepPrime. Only difference is that I use PhotoLab 6 but the general processing should be the same.
As you can see, the GPU is used 100% but only for some of the time and the CPU is never stressed to its max.
Now, the same images but with Prime, which utilizes the CPU
This screenshot was taken while still processing. As you can see no GPU usage (the low spikes are not from PL) but 100% CPU usage.
As for your second question: will the RTX 4080 greatly reduce the running time? That depends: if you use DeepPrime it will shorten the time as the RTX 4080 is a lot faster. This difference will be even bigger if you use the newer DeepPrime XD in PhotoLab 6.
If you don’t use DeepPrime, getting the RTX 4080 will only drain the money from your wallet.
There’s a benchmark sheet on this forum mentioned here
The RX 5600 XT is slightly slower than GTX 1070Ti. If you look at lines 170 and 171 in the Excel sheet, you’ll see the GTX 1070Ti and the RTX 4080 processing the same images. Please note that they processed these images in PhotoLab 6 using the new DeepPrime XD but it gives you an idea of the performance increase. It’s up to you if you find it worth the investment 