Context switching costs are slightly higher for OS threads, but the real bottleneck is in the physical memory usage. An OS thread will start with an entire 4kb

Context switching costs are slightly higher for OS threads, but the real bottleneck is in the physical memory usage. An OS thread will start with an entire 4kb (page size on most OSs) for its stack and grow by 4kb as needed. A virtual thread will often have <1kb physical memory used, to start. In a program with lots of small OS threads, that's quite a bit of wasted space.