You’re right that finding available services can be challenging when new to the environment. The Built-in Services Reference is an excellent resource for browsing what’s available.
The ps.util.system:getTimeMillis is one possible approach. Call that at start. Then call it again for the end time. Then use pub.math:subtractInts (in WmPublic) to get the elapsed time in millis.
Note that the getTimeMillis service is from the PSUtilities package, and is not an out-of-the-box package installed by the Installer. It is a separately downloaded package you can get from Advantage.
#webMethods#Integration-Server-and-ESB#Flow-and-Java-services