Nearly all frictional interfaces strengthen as the logarithm of time when sliding at ultra-low speeds. Observations of also logarithmic-in-time growth of interfacial contact area under such conditions has led to constitutive models which assume that this frictional strengthening results from purely time-dependent, and slip-insensitive, contact area growth. The main laboratory support for such strengthening has traditionally been derived from increases in friction during ‘load-point hold’ experiments, wherein a sliding interface is allowed to gradually self-relax down to sub-nanometric slip rates. In contrast , following step decreases in the shear loading rate, friction is widely reported to increase over a characteristic slip scale, independent of the magnitude of the slip-rate decrease-a signature of slip-dependent strengthening. To investigate this apparent contradiction, we subjected granite samples to a series of step decreases in shear rate of up to 3.5 orders of magnitude, and load-point holds of up to 10,000 s, such that both protocols accessed the phenomenologi-cal regime traditionally inferred to demonstrate time-dependent fric-tional strengthening. When modeling the resultant data, which probe interfacial slip rates ranging from 3 μm/s to less than 10^-5 μm/s, we found that constitutive models where low slip-rate friction evolution mimics log-time contact area growth require parameters that differ by orders of magnitude across the different experiments. In contrast, an alternative constitutive model in which friction evolves only with interfacial slip fits most of the data well with nearly identical parameters. This leads to the surprising conclusion that frictional strengthening is dominantly slip dependent even at sub-nanometric slip rates.