Why is a fast dictation considered to be difficult?

The time taken to transcribe an audio depends on the number of words typed. The time taken to type out a dictation/audio which has very fast speaker/speakers on it will be much more than the average. Hence, if the speed is 20% above the average, ie. above 12 lines per audio minute, then we deem the audio to be difficult as it will take the transcriber that much more time to type out the content.