That is fine.
Slower charging is preferred, because it is better for the battery.
Fast charging shortens the life of the battery.
The slowest charge would be from a USB port from a computer or a laptop.
It may not carry enough current to charge and use the device at the same time, and will be incredibly slow, but build up a negligable amount of heat.
Normal charging, 'preferred' in your case, is a standard rate that will charge while the device is used, and should build up a nominal amount of heat.
Fast charging builds up a ton of heat, and charges quickly- although not always thoroughly.
This will wear the battery out quicker than the other charging speeds.