Hiya, Ultimate Doom is a great place to start speedrunning. If you don't already have one, you should get a good sourceport. The currently recommended ones are prboom+ and crispydoom. The best way to practice speedrunning is to just try running through the level again and again, or watching other peoples speedruns on the leaderboards (which I do all the time). You can learn so much just by seeing what other people do. But of course those two things are probably obvious, so if you want more general times, learning SR40 and/or SR50 (which is much more difficult). Both are tactics to move much faster through the levels. SR40 is achieved by running while also pressing [forward] and [strafe right/left] at the same time. So for example if you're using wasd, shift+w+d / shift+w+a. SR50 however is much harder to pull off, and I suggest looking it up for yourself. Cyberdemon531 posted a video explaining it some years ago. Another general tip is to always move back from doors when you open them, that way you can gain momentum. You see this constantly in speedruns on the site. Also if you're doing 100% runs, it's important to know that lost souls don't count towards the kill%, so you can skip them.