Did you report information about the system affected? What filesystem is it on? If it's ext4 does it have extents enabled?
I think on ext3 or ext4 without extents it won't have any benefit but it shouldn't really be any slower either since the libc implementation is very similar to what we used to do. I wouldn't expect the benefit from it to have a huge effect either. The main benefit is actually the reduced fragmentation so it would be pretty dependent on the filesystem it's on anyways. Avoiding stalls when creating new wal files is also nice but shouldn't happen often enough n your benchmarks to cause an effect on the average numbers.
