I haven't been able to find documentation from Microsoft as to which algorithm DirectWrite uses, but if it's the same as the algorithm Direct2D uses for general path rendering it has an expensive CPU-side tessellation step first and so has the typical drawbacks of long setup times. I wouldn't be surprised if DirectWrite does the actual path rendering on the CPU, like Skia does in typical configurations.

Core Text (really, Core Graphics) renders paths on CPU. I benchmarked it and it generally performed a bit worse than stb_truetype.