You can easily calculate that if the reaction would appear at every collision, at every encounter between the two ions, then... and if the concentration of the ions were 1 molar, 1 mol/l, then the half-time of such a reaction should be only a fraction of a nanosecond, that means something like 10^-10 seconds. And now the difficulty is, in order to measure the rate you have to bring together the two reactants, let's say the barium ion and the sulphate ion, by mixing two solutions, one containing the barium the other containing the sulphate. Or, in the case of neutralisation reaction, taking an acid solution which has an excess of protons and a base solution, basic solution, which has an excess of hydroxyl ions, and so that the proton and the hydroxyl ion can combine to a water molecule.

Now the difficulty is the mixing has to be complete, otherwise what you see is not the reaction, but is the mixing of the two substances, and that cannot easily be effected, not even in a sophisticated apparatus in less than a millisecond, less than a thousandth of a second. What happens is that you have to put these solutions under high pressure together and make a turbulent mixture of the two and then it flows down through an outlet, and that can't be effected in less than a millisecond. In fact, to get down to a millisecond was already a great success by Hartridge and Roughton in England. They developed a flow device in which they... before them the shortest times one could measure were in the range of seconds to minutes, and they managed with this device, flow device, to get down to a thousandth of a second, to a millisecond, and that was about the... and one thought, well, for a reaction to trigger you have to bring together the partners and that can't be effected in a time shorter, therefore he called it immeasurably fast reactions.