PMIC limits charging current to 1A . So even if you use 10A charger , battery will charge at 1A. But it charges faster on higher voltages up to 5.5V ( Voltage * current = power . 1A*5V=5W , 1A*5,5V=5,5W). So its ok to use chargers up to 5,5V and 3A - 4A
I think main reason of using weak adaptor is battery. Because most of lumia phones with 3.8V battery have Samsung SDI battery and those batteries are worse than sony batteries which is used by nokia in older devices like 1520 1320 , 820 ex . Samsung SDI batteries lose their capacities very fast .And another thing is i tried charge 640XL battery externally with 1.5A current , battery suddenly started getting warm. So thats why microsoft wanted to limit charge speed.