Record single star speedruns with an In-Game Timer (IGT). To add the timer, copy the relevant cheat code from the site below:

https://sites.google.com/site/sm64gameshark/codes/timers

On this site you will also find the additional "Make timer display centiseconds" code. Use this code in addition to the "Manual timer" code. (Reset the timer with the L button.)

You can also find variations of these codes that automatically resets the timer, shows the timer inside the castle, changes the timer position, etc.

Please note that the timer does not display all 100 centiseconds for each second. If the timer were at 10.03 seconds for example, the next time it would display would be 10.06 and not 10.04 seconds.