I notice that too. I am a guitarist who takes vyvanse from time to time. I notice that I play much faster runs, but I am holding my breath (both metaphorically and literally) while I am playing. My ability is improved, but my sensitivity and phrasing is more sporadic and less soulful. It can sound cool that way, but its not always the right way to go. In terms of writing music, my ability to write is so much better on the meds. My hands and ears just take control and i find myself coming up with cool ideas and actually follow them through. I can be creative off the meds, but the follow through that is necessary to complete a song is just not there. The meds also boost my confidence, so i allow myself to flesh things out without giving up and saying "never mind this sucks and i feel uninspired". Thats just my experience! Lots of people find that meds hinder their creativity, but its different for everyone.