usage.c:24 says the--fast alias for -i u10000 (10 packets for second)\n"; however
parseoptions.c:505 says usec_delay.it_interval.tv_usec = 100000;

If I understood correctly, these numbers assume packet submission time is negligible; hence the usage.c or parseoptions.c disagree - both should be 10 packets for second (and -u100000) or 100 packets for second (and -u10000).

Thanks for hping!