Hacker News Reader

The rendering itself isn't that large of a task and it'd all be done on the POS which has more than enough power to draw a couple circles. There's very little processing done on a thermal printer basically it's just decoding and drawing exactly what the source system tells it to. At most I'd expect it to add a second or two for the POS to do the calculations and send it to the printer.

That's the computer's problem, not the printer's. If rendering time really becomes an issue, it can start the rendering process while the humans are twiddling about with card reader terminals and making small talk.

The print time is proportional to the number of lines with pixels

Watching the fast thermal printers in stores it's fluid enough it might just be directly related to length.