Qmail needs 59 seconds to deliver 100 SMTP messages to one local recipient.
Default configuration, concurrencyremote = 100.

/usr/bin/time ./smtp-source -s 5 -m 100 -t wietse fist
       41.45 real         0.07 user         0.21 sys
Jun  8 12:17:20 fist qmail: 865786640.494072 new msg 39901
Jun  8 12:18:22 fist qmail: 865786702.301087 end msg 39982
Total time: 62 sec

/usr/bin/time ./smtp-source -s 10 -m 100 -t wietse fist
       26.50 real         0.10 user         0.22 sys
Jun  8 12:14:49 fist qmail: 865786489.089492 new msg 39901
Jun  8 12:15:48 fist qmail: 865786548.316898 end msg 39928
Total time: 59 sec

/usr/bin/time ./smtp-source -s 20 -m 100 -t wietse fist
       21.15 real         0.08 user         0.30 sys
Jun  8 12:19:18 fist qmail: 865786758.939755 new msg 39903
Jun  8 12:20:16 fist qmail: 865786816.739912 end msg 40031
Total time: 58 sec