Finger snapping

[mdshimazu]

There is an ostinato in the piece I'm playing right now that's supposed to be marcato, so my teacher has me doing this technique where you place your finger just above the note and do a kind of motion where you snap your finger towards your palm to produce a marcato sound. It works, but too slowly. Is it a matter of just practicing it over and over or something else?

I've also been practicing scales using this technique, they too are rather slow.