Wow! I’ve been using QuickTime as long as there has been a QuickTime, and I never knew about the hidden J/K/L controls. Apple certainly didn’t document those keyboard commands very well. Thanks for the tip!.
The K+L combo really isn’t doing slo-mo playback, per se. It’s actioning a repetitive single-step (frame by frame) series, the same as you get by holding down the right-arrow key. (right-arrow single steps forward, left-arrow single-steps backward).
Now that I think about it, the old QuickTime Player Pro (from 2010 or so) had a hidden control panel that provided adjustable playback speed. I used to have QT Pro installed on an old Mac. I should see if I can find it and try running it on a current vintage Mac.