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The question is which physical process sets this floor: the chain moving through water, or friction inside the chain itself.","posed_since":"2004","precise":"For ultrafast folders (villin headpiece, WW domains; folding times $\\sim 1 \\text{ to } 10\\ \\mathrm{microseconds}$), determine the prefactor $k_0$ in the Kramers rate $k = k_0 \\exp(-dG/k_B T)$, with the empirical estimate $\\tau_{\\min} \\sim N/100\\ \\mathrm{microseconds}$ for $N$ residues (Kubelka, Hofrichter and Eaton, 2004), and separate solvent from internal friction through the viscosity dependence $\\tau = a\\eta + b$ at fixed stability ($\\eta = \\text{solvent viscosity}$). An answer gives $k_0$ and the internal-friction term $b$ with their molecular origin (backbone dihedral barriers, side-chain contacts) and reproduces measured $\\eta$-dependences.","problem_ref":"4a9084622d681c20bd8298a4e37cbf2b8d4e1775d949304dc4c12b2a99c82a0d","references":"","settled_by":"Viscosity-dependent single-molecule transition-path and folding-time measurements on ultrafast folders matched by all-atom or coarse-grained simulations that identify the friction source.","status_note":"Single-molecule measurements since 2012 bound transition-path times at microseconds; the molecular origin of internal friction is still disputed as of 2026, to this survey's knowledge.","title":"What sets the folding speed limit and internal 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