I would say your problem is that you're using an apple product but that doesn't really solve anything. Unity usually only uses 30 to 50 percent of the GPU, at least on my personal computer, with HDRP. So low framerate would be expected, but if your PC really is at 100 percent utilization it probably is actually because of the device itself. Apple products don't really have good hardware, built-in graphics on anything is usually pretty terrible. Though still not knowing what your volume profile has on it I have no idea if it is because you're on an apple product or if you have some feature on that eats up performance. Like ray-tracing or volumetrics.
Physically based sky uses a lot of performance, but also if minimizing the window decreases the resolution then the performance is going to get a lot better. Looking at your stats window it looks like you probably have free aspect on, so a smaller window is going to be a lower resolution than when it's maximized, so of course the fps with be low. What resolution is the screen on your device?
