This is not really speed limits, this is the speed you could drive on this roads, so saying you could drive 16kmh on primary, that means you have obstacles.
Decreased priority section is actually same "speed" multiplicator, but it is made separate to take into account details. So I think correct way of defining routing.xml would be to have all different roads here and leave only multiplicators in priority section.
For instance for primary 16 * 0.85 = 12.8 * 1.

Previously routing was using it as a special multiplicator, but now taking into account our "previous" experience, it is better to define most speeds and have priority mostly 1 and in exceptional cases low or tweaked by parameters.