Elchimango: Keep in mind that the larger granules of the slower-burning powders need more heat for ignition than the finer granules of fast stuff.
Also, the volumes of the charges of most rifles are greater than for most pistols.
Most Magnum charges are of the larger/slower powders.
So, a Magnum cartridge's powder charge needs more "Oomph" to get a uniform and complete ignition than does a smaller cartridge, whether rifle or pistol. Likewise for a rifle cartridge as compared to a pistol...
Primers are designed to function through a specific dimension of flash-hole, and enlarging the cross-sectional area of a flash-hole can lead to a more explosive type of ignition of the powder charge, giving possibly-damaging pressures.
Primer manufacturers deserve credit for the amount of uniformity between brands, as well as uniformity of their primers' ignition characteristics...Between brands, there is a certain variation in temperature and/or length of time of the mini-explosion--as you say, nano-seconds.
Hope this helps, Art