  The growth of the spin-glass correlation length has been measured as a
function of the waiting time $t_{\mathrm{w}}$ on a single crystal of CuMn (6
at.\%), reaching values $\xi\sim 150$ nm, larger than any other glassy
correlation-length measured to date. We find an aging rate
$\mathrm{d}\ln\,t_{\mathrm{w}}/\mathrm{d}\ln\,\xi$ larger than found in
previous measurements, which evinces a dynamic slowing-down as $\xi$ grows. Our
measured aging rate is compared with simulation results by the Janus
collaboration. After critical effects are taken into account, we find excellent
agreement with the Janus data.
