With no sleep enabled, I consistently get extremely low send/recv times:
$ gcc -o udp_loopback_test_c udp_loopback_test.c && ./udp_loopback_test_c
[1.0] send: 4.93us recv: 7.41us
[2.0] send: 4.68us recv: 7.04us
[3.0] send: 4.86us recv: 7.58us
[4.0] send: 4.79us recv: 7.60us
[5.0] send: 4.88us recv: 7.03us
[6.0] send: 4.70us recv: 7.57us