Adaptive settings is essential for lower-end graphics hardware and CPU-based volume rendering. It measures rendering time and degrades rendering settings to achieve the desired interactive refresh rate. It works very well for CPU-based rendering, but since GPU-based rendering performs a lot of processing in background threads and on the hardware, rendering time measurement is not a good estimation of how overloaded the rendering pipeline is.
Probably we could use “Normal” quality for GPU-based rendering by default, but unfortunately, quality mode is defined in the view node and not in the display node, so it would not be easy to change it cleanly.