first step was to take advantage of the tunables available:
especially delaytime makes a lot of difference, 5 seconds on average.
next step was to upgrade to the beta version, which improves this quite a bit! thanks, Keke.
it will still wait for a quite long time between/after each command, on average 100 ms, which adds up. I reduced the poll interval to 25 ms but increased the number of timeouts correspondingly except in the case where it just checks for unexpected output from the modem.
at this stage sending an SMS takes ~8 seconds. the last step is to replace the FalcomTango modems with more modern modems which respond *much* more quickly to each command.