@RMD I did not use fillers (uhm, eehh,etc), and just left them as Silent pauses. Just make sure the pauses are less than 3 seconds, so it will continue recording.
For retell lectures, describing graphs, and those test types that require long speaking answers, i recommend to answer those questions until the recording limit. Use all the 40secs.
For the simple question and answer speaking tests, 1 word is enough and don't answer too quickly and make sure it has started recording.