8.0.702 on macOS 10.14.6 :
Pike bertrand$ for i in `seq 10`; do ./test_callout.pike; done took 0.006 ms took 0.007 ms took 0.006 ms took 0.006 ms took 0.006 ms took 0.006 ms took 0.006 ms took 0.005 ms took 0.006 ms took 0.006 ms
The default backend in MacOS X uses kqueue(2) which uses a timespec for the timeout, and thus doesn't have the granularity issues that the affected poll(2) and epoll(2) et al have (which have a timeout granularity of ms).