im getting into speedrunning and my question is: what is th best way to add a timer to your mobile runs?

You don’t really need a timer for Speedrunning, but if you want to see your time while your running a game, here are some options. 

Android: 
[Mobizen](https://play.google.com/store/apps/details?id=com.rsupport.mvagent)

[XRecorder](https://play.google.com/store/apps/details?id=videoeditor.videorecorder.screenrecorder)

[Stopwatch](https://play.google.com/store/apps/details?id=de.jentsch.floatingstopwatch)
iOS: 

iOS has a built in screen recorder 

[Floating Timer](https://apps.apple.com/us/app/floating-clock-timer-stopwatch/id1545752052)

Best speedrunning timer