ChangePasswordUtil takes 12-14 seconds to change a password. Its first invocation takes 25-30 seconds (additional time spent creating embedded service-defs). During the initial start of Ranger Admin there are 4 passwords being changed (admin, rangerusersync, rangertagsync, keyadmin), total time for password change is ~74 seconds.
Based on ChangePasswordUtil's logs, most of the time is spent on setup, and the actual password change is very quick.
So it will be better to change multiple passwords in one request (by sending multiple users passwords config in single run of ChangePasswordUtil)