Numerous students have approached me regarding the seeming correlation between playing faster and the shorter the note value. However, this correlation is only perceptual, providing that the only way the performer could actually play faster is by altering the tempo.
Notes that are shorter such as the sixteenth note, for example, are not performed faster than longer notes such as the half note.
Because a note is a duration of sound, the duration of shorter notes are just simply sustained for a shorter period of time. For this reason, as the listener hears a succession of sixteenth notes in comparison to a succession of half notes, the audible perception is that the performer is playing faster. Nonetheless, this is a variance in sustained sound and not speed.