I typically use TTS near 2.5x (I turn it up when I'm alert and down when I'm tired.) It's definitely a learned skill; a few years ago I started at 1x and struggled even with that.

Every couple of months, take a moment to reflect on your comprehension. Is it currently easy for you to understand the audio? If yes, then crank it up a little bit until it's noticeably more difficult. Repeat this process periodically over a year or so and before you know it, it'll be set pretty damn quick.