So, your GPU usage is dropping because your CPU is getting hammered.
In addition to deleting the WUs that cause the reduced performance on the ground, I would still very highly recommend reducing traffic and LOD settings, and increasing graphics quality and render scale settings to see if your CPU latency improves.
My procedure for doing this in a logical, measurable way is in this thread. It’s a lot to read, but worth the time:
Once your CPU has some breathing room, it will be able to feed your GPU again, and your GPU utilization will increase.