Tognazzini, Maximizing Human Performance; Omanson et al. 2010, Comparison of Mouse and Keyboard Efficiency; Card, Moran & Newell 1980, Keystroke-Level Model.
I seemed to recall that Tog made that point back then. Note that both Tog‘s text and Keystroke-Level are from the very early days of GUIs, so, grain of salt.
Omanson, OTOH actually contradicts them a bit: expert users are generally faster with well-learned shortcuts – a widely-held view point and the whole reason for shortcuts.