UPDATE Jan 2022: GPU utilization varies widely, and for unknown reasons across machines. On the 2023 Mac Pro M2 Ultra, %GPU readout is 0%, even as ~25% GPU history graph is shown, with modest CPU utilization.
The Adobe Camera Raw Enhance Details is the most important raw file conversion feature in years, applied as a preprocessing activity to the camera raw file. I apply it to all my raw files for improved image quality. But it is very slow, taking up to 5X as long as converting the raw file!
Unlike raw file conversion which has zero GPU usage and modest GPU usage for Photoshop Filters, the Enhance Details feature is GPU-constrained, so a faster GPU should considerably speed up Enhance Details. As of mid 2019, that’s about the ONLY Photoshop operation where a GPU has much relevance—choosing more and faster CPU cores should come first before spending on the GPU. Thus it is doubtful that an eGPU makes any sense at all except for machines with integrated graphics (only).
Below, the GPU on the 2019 iMac 5K is pegged out at 100% utilization while converting raw files. There is some CPU usage, but any 4-core Mac should handle that easily—the key is the fastest possible GPU.