alt Hacker NewsThat's because it's logical that implementing network capable segmentation and flow control is more costly than just moving data with internal, native structures. And looking up random benchmarks yields anything from equal performance to 10x faster for Unix domain.
It wouldn't surprise me if inet sockets were more optimized though and so unix sockets ended up slower anyway just because nobody has bothered to make them good (which is probably why some of your benchmarks show equal performance). Benchmarks are important.