How does one exactly filter it to only fetch those that finished under a minute? I also tried using aggregates since groupby userId was possible there, however, I cannot find the right filter to range the call duration.
Looks like the API explorer erroneously lists metric as valid filter option when it's not, so short answer is your only option is to do your grouping entirely manually in whatever language you're hitting the API from.
The APIs are not really built for sophisticated combinations of logical criteria.