alt Hacker Newsits slower than text mode. basically you can print anything as long as you can convert it to monochrome bitmap before sending. But the thermal printers have anotehr mode which prints extremely fast if your data is textual with rudimentary formatting like order slips.
I believe the speed also depends on how many activated dots there are per line in the image, as thermal print heads often have a limit to the number of elements that can be activated at once.