Daniel Ingram:
How to measure noting speed. Start by counting "one one thou sand", which, if you say it at a normal pace, should take a second. The syllables number four, so that is 4Hz, one pulse or syllable per every 1/4th of a second, or four syllables per second.
Ok, now get musical with this, if you have some musical sense, as this makes counting frequencies a lot easier. You can make it two triplets and get 6Hz by saying or thinking "one and one thou ow sand" in a second.
Try doubling the pulse of each syllable, which could be done various ways, such as "oneone oneone thouthous sandsand", and still think this in a second, so you now have 8Hz. You could try tapping this with your hands, and you will notice that it is pretty easy to tap 8 times per second. Making them triplets gets you 12Hz, still pretty tappable. Making them quadruplets gets you to 16Hz, getting hard to tap for most people, but your mind can easily go faster than your fingers, so still mentally pretty doable. Getting faster than quadruplets requires some diving into the experience and likely some dedicated practice. I tend to just double the quadruplets and get 32Hz, which then allows some extrapolation of frequencies in the range between 16Hz and 32Hz.
Speed 32Hz up either feeling that pulse and just speeding it up a bit, or starting with five syllables (which, multiplied by eight gets you 40), will get you a sense of what 40Hz is like, which is zipping fast. There is an app called Audio Function Generator (Pro), which can create wareforms of various types and is great for getting a sense of how fast these pulses are. I have no financial relationship to this app, BTW. If you set it on the waveform farthest to the right (looks sort of like a very flipped sideways "Z") and put in the frequency you are interested in, you can start to learn how fast these are. There are other sound generating apps and devices that will also work.
You can listen to each of them and get a sense of how fast each pulse is, and then, after doing that, if you have an ear for it, you can more rapidly get a sense of how fast pulses of experiences are without having to do all the counting, though the counting is really useful in the beginning when trying to learn how to do this.
Like so many things that initially seem daunting, this just requires practice.
I remember when I first started playing scales on my guitar, and it took me seconds to get my fingers to move to the position for each note, and I felt like I had some sort of movement disorder, I felt so clumsy, but now I can play scales faster than I can easily see my fingers hit each note, and yet they do hit them, and I can hear my fingers hit them, but it is now faster than my eye can follow when I get up to maximum speed. Learning to play scales fast took me a lot of practice, many, many hours over years. This process of leaning to count frequencies actually came more easily than playing scales fast, but I still found a lot of work.
Be patient and start with the slower pulses and build up until this is natural for you, if you wish to play the "how fast is my mind going" game, which is a pretty fun game, if you are a serious dharma phenomenology geek like I am.
… I have only achieved discrete, countable/extrapolateable speeds past about 40Hz or so in very rare, fleeting moments, usually on the tale end of the ramp up of an A&P-style event, but have easy access to stuff in the mid 20Hz range.
is what you might find in Youtube. You probably have already find this Beat X Hz videos like
Low Beta 15 Hz or
Binaural Beat Session (15 Hz), but the interesting thing is to play them simultaneously to hear something similar to rotorblades (though they have nice tones, while the one I hear is more deaf like tone).
. And thanks for reminding me of Ingram's lowtech method 
.