How the flow is worked out
Each extruded line is treated as a rectangle: layer height times line width gives its cross-section in mm², and multiplying by the speed in mm/s gives mm³ of plastic per second. Slicers that model the rounded sides of a line report a slightly lower figure, so this one errs on the safe side.
The flow limit belongs to your hot end and to the filament. Hot end makers publish it, often for PLA, and a stiffer or hotter material usually gets less. Dividing the limit by the cross-section of the line gives the fastest speed that stays under it.
A 0.2 mm layer at a 0.45 mm line width is 0.09 mm² of plastic. At 200 mm/s that is 18 mm³/s.
For the reasoning behind the figures, see the nozzle clog guide.
Questions
What is volumetric flow in 3D printing?
The volume of plastic the hot end melts each second, in mm³/s. For one line it is roughly the print speed times the layer height times the line width.
Why do prints come out thin when I raise the speed?
Once the flow asked for passes what the hot end can melt, the plastic cannot keep up and lines come out under-extruded. Lower the speed or the layer height until the flow sits under the hot end maker's figure.
