The feat is that they spin all the threads of life, fate and death all at once. Clotho states that it is from her threads that all life on Earth is born. The scans are on that blog above.
Assuming "all life on Earth" only refers to non-microscopic life/only actual surface life, the number of which is >10 quintillion, and that each thread is about five meters in length, you have 50 quintillion meters. Then you divide that by the fact that they're said to do this in the precise instants, such as the instant in which a life ends and the brain function ceases, and you have a minimum timeframe of 0.1s for neural speeds.
That is, needless to say, so high into MFTL that it isn't funny.