1.5x or faster speedups, turn on CC for the unintelligible speech. I really appreciate it when there's a transcript available in addition to the captioning.
Taking notes / creating Anki cards helps with focus.
I use noise-canceling headphones (inexpensive from Taotronics) and the DFX plugin [1] for audio clarity. The plugin has a Dialog Boost setting intended for transcription. It goes a long way to improve muddy audio.
In case some are wondering, "Video Speed Controller" extension can speed and slow down videos by pressing "d" or "s", respectively as much as you want with steps of 0.1x, instead of the non useful Youtube click and select a speed with a quantum of 0.5 and a max of 2.
If a talk is 40 minutes and there are no slides available and the author goes through the slides in the video, I'll play the video at 10x or more just to check out the slides and see if the talk addresses the points I'm interested in. 40 minutes at 10x or faster is around 4 minutes. It helps me decide and avoids me wasting 40 minutes on the wrong talk.
Taking notes / creating Anki cards helps with focus.