It depends on how many high dpi bitmaps at full depth are kept. Uncompressed HDR 4K gets large pretty quickly.
And then it's not one bitmap, but multiple layers - backgrounds, rendered text...
And this is where I don't know what to think because "keep it all in vRAM" is clearly a terrible algorithm, and I have faith in the macOS developers, and yet my data suggests that this is exactly what they're doing. I don't see things swap out when there's memory pressure; it just keeps on allocating until it's full, then spills over to system memory and makes everything slow.
Counterintuitively, the iGPUs have an advantage here. They always run from system memory but they have a faster path to system memory -- they don't need to operate over PCIe -- so (a) you don't have a large perf gap between the 'full' and 'not full' cases, and (b) spillover doesn't hurt. You use a lot more RAM, but you can buy more of that. I can't easily add RAM to my GPU. At best I can double it, and that just means 70 Chrome tabs instead of 35.
I'd love to hear from someone at Apple who's seen the internals of this.
I think successive software updates have made the situation marginally better - but I might have just acclimatised to it.
I’m very curious what the situation looks like with arm laptops when they launch. How will the graphics performance of an A14x chip compare to my situation at the moment? I assume they’ll be faster than intel iris graphics but slower than an egpu; but it’ll be very interesting to see!