MX needs 24 seconds to deliver 100 SMTP messages to one local user.
smtpd/local-delivery process limit = 100

/usr/bin/time ./smtp-source -s 5 -m 100 -t wietse@fist.porcupine.org fist
       10.47 real         0.12 user         0.16 sys
Jun  8 14:45:25 fist mx:smtpd[19432]: connect from spike.porcupine.org(168.100.1
Jun  8 14:45:49 fist mx:local[19444]: 085788: to=<wietse@porcupine.org>, relay=l
Total time: 24 seconds

/usr/bin/time ./smtp-source -s 10 -m 100 -t wietse@fist.porcupine.org fist
        9.10 real         0.06 user         0.26 sys
Jun  8 14:46:42 fist mx:smtpd[19443]: connect from spike.porcupine.org(168.100.1
Jun  8 14:47:06 fist mx:local[19446]: 085792: to=<wietse@porcupine.org>, relay=l
Total time: 24 seconds

/usr/bin/time ./smtp-source -s 20 -m 100 -t wietse@fist.porcupine.org fist
        9.84 real         0.09 user         0.28 sys
Jun  8 14:48:03 fist mx:smtpd[19458]: connect from spike.porcupine.org(168.100.1
Jun  8 14:48:28 fist mx:local[19479]: 085795: to=<wietse@porcupine.org>, relay=l
Total time: 25 seconds