On one listening to a performance of Albinoni's Miserere he went home and transcribed it, allegedly note perfect. Surely that would have been easier than actually having to come up with a few ideas (though given his phenomenal output, he didn't have too much trouble with that either).
Well my ears obviously aren't that well developed..
I can listen to and transcribe - but since I can't memorise the aural data from one listening I would have to go over it many times to get it note for note
If I "improvise in my mind" there is no need to memorise, its just a free flow. And if I go over a passage, it doesnt have to be identical because the notes are my call. This is far easier than transcription as far as speed, considering that I can just spurt out music in real time or faster if I don't have to be able to write it out aswell. The time it takes to write it out severely interupts the creative flow for me, which is why I rarely do it.
^The time is taken up if I start getting picky about refining the composition, and being able to play it convincingly, which is why i said assuming quantity over quality.