What is Diffusion and activation control, cage effect?

Example 12.16 (Speed limits in water and in hexane)

At $298.15\,\mathrm{K}$, water ($\eta = 0.890\,\mathrm{mPa}\,\mathrm{s}$) gives $k_d = 8 \times 8.314 \times 298.15/(3 \times 0.890 \times 10^{-3}) = 7.4 \times 10^{6}\,\mathrm{m}^{3}\,\mathrm{mol}^{-1}\,\mathrm{s}^{-1} = 7.4 \times 10^{9}\,\mathrm{L}\,\mathrm{mol}^{-1}\,\mathrm{s}^{-1}$; hexane ($\eta = 0.298\,\mathrm{mPa}\,\mathrm{s}$), $2.2 \times 10^{10}\,\mathrm{L}\,\mathrm{mol}^{-1}\,\mathrm{s}^{-1}$. No bimolecular reaction between neutral molecules of ordinary size is faster; measured constants near these values (the quenching of excited states, the recombination of radicals) are diffusion-controlled. Ions of opposite charge attract and can go faster.