Carmack is a great empiricist, meaning he pays a lot of attention to how things work in a running system. All of his engines provide rich facilities for record and replay as well as running benchmarks.

timedemo <demo file> uscache

The DOOM3 source is here https://github.com/id-Software/DOOM-3

There are lots of facilities for recording and playing back demos. The author could record their own gameplay and play it back in realtime vs running timedemo (which plays back as fast as possible).

https://github.com/id-Software/DOOM-3/blob/master/neo/framew...

    2865: cmdSystem->AddCommand( "recordDemo",  "records a demo" );
    2866: cmdSystem->AddCommand( "stopRecording", "stops demo recording" );
    2867: cmdSystem->AddCommand( "playDemo",  "plays back a demo",
    2868: cmdSystem->AddCommand( "timeDemo",  "times a demo", 
    2869: cmdSystem->AddCommand( "timeDemoQuit",  "times a demo and quits", 

https://www.youtube.com/watch?v=CLA42q3myCg

https://www.youtube.com/watch?v=9KIhRmdz2cI