no,In my experience it doesnt slow down your playing.
left middle finger plays C, LEFT index plays G, left ring plays E.
This sort of thing is useful for song accompaniment,you can then play a C add an E,and add a G,While holding on the previous notes.
or you can play similiar to a guitarists dum ching.
C Then on the off beat play EG together,I know this as vamping,rhythym 1 . 2.this is impossible using the one finger for two button system,that is why it is limiting.
It is just a question of practice.
when playing tunes I often use the octave note to accentuate a note,or a fifth below like a fiddlers double stop,but I nearly always use two different fingers.http://www.dickmiles.com