I wonder have you got the toner save button on.... or check in the
settings in the printer monitor window, usually a setting that allows
you to lighten and darken text.
The text usually start to fade as the toner runs out, some Samsung
laser are easy to refill, if it is because the toner is running out a
little trick you could do...... is to take the toner unit out, hold some newspaper over the drum
end and give it a couple of shakes.

Thanks your answer! And The original cartridge or the compatible cartridge， which toner cartridge is cost-effective ?