Im not sure if the client defaults to using the GPU, cos Im lagging quite badly with potato graphics. I suspect it may be because the client is pushing the bulk of the work to the CPU, and want the game to use the GPU instead
If you have multiple GPU’s, you can select which one. However, the issue in your case is likely be caused by something else, such as your GPU overheating and throttling (especially if things used to run fine, and then stopped recently). Use HW monitor to see what your GPU/APU utilization and temps are (you can look up max temp for your GPU/APU online).
Anyway, I’ve got to work, but I’ll check in later today. Just at me or reply to me if you have more questions or need more help.